A Virginia man has been charged with second degree murder after fatally shooting a teenager who was filming a prank for TikTok known as “ding dong ditch” with two friends around 3 a.m. on Saturday, according to court records and local authorities.

The Spotsylvania Sheriff’s Office responded to a reportof a resident firing shots during a burglary, and found two teenagers with gunshot wounds, the office said in a statement. One of the teenagers, Michael Bosworth Jr., 18, later died of his wounds. The second person was treated for minor injuries, and a third person in the group was unharmed, the sheriff’s office said. The two friends with Mr. Bosworth were both under 18.

The teenagers had been in the neighborhood to make a TikTok video, one of them told investigators in an affidavit filed in Spotsylvania Circuit Court. A “ding dong ditch” prank involves ringing doorbells or knocking on the front doors of houses before running away, and has become popular fodder for social media videos.

“The juvenile advised it’s something that people are doing to put on TikTok,” the affidavit said.

The group had knocked on a few doors in the area, one of the teenagers told a detective, adding that they were not familiar with the neighborhood. They were running away from a residence when they were shot, according to the affidavit. At least one video showing the teenagers doing the prank was still on one of the friends’ phones, the affidavit said.

The authorities arrested Tyler Chase Butler, 27, of Spotsylvania County, on Tuesday on charges of second degree murder, malicious wounding and use of a firearm in the commission of a felony, the sheriff’s office said. He was being held at Rappahannock Regional Jail on no bond, it said.

Mr. Bosworth was a senior at Massaponax High School in Fredericksburg, Va. The high school, which was set to hold its graduation for seniors on May 13, sent a message to the school community that counselors would be available to help grieving students.

A spokesman for the Spotsylvania Sheriff’s Office, reached by phone, declined to comment further. A lawyer for Mr. Butler did not immediately respond to requests for comment. G. Ryan Mehaffey, the Commonwealth’s Attorney for Spotsylvania County, declined to comment but said a preliminary hearing had been scheduled for June 18.

This style of prank has led to tragedy in the past. In 2020, a man in California crashed into a car of six teenagers, killing three of them, after they played a similar prank on him. He was sentenced to life in prison in 2023.

On Tuesday, a group of students gathered on the football field at Massaponax High School to remember their classmate, according to a video shared by an Instagram account run by students from the school. They shared memories about Mr. Bosworth and wrote messages on balloons before releasing them at sunset.

Jonathan Wolfe and Michael Levenson contributed reporting. Susan C. Beachy contributed research.

In September 2006, Mark Zuckerberg, the chief executive of Facebook, described what made his social network special.

“Facebook is about real connections to actual friends,” he wrote in a company post.

Two decades later, that description is at the center of a landmark antitrust trial against Mr. Zuckerberg’s social networking empire, now called Meta, and whether it illegally stifled competition. In essence, the trial has raised the question of whether social networking is simply about connections to friends and family, or whether it is something more.

The Federal Trade Commission, which is prosecuting the case, has tried to narrowly define social networking as a service that links friends and family. Under that definition, Meta would really compete only with Snap, the maker of Snapchat, which it dwarfs in size and users. But Meta has argued that all social media companies count as rivals, especially TikTok and YouTube, which would mean that competition was more abundant.

“The friend part has gone down quite a bit,” Mr. Zuckerberg said in testimony at the trial last month, downplaying his words from 2006.

The opposing definitions of social media in the case — Federal Trade Commission v. Meta Platforms — illustrate how much social networking has evolved over more than a decade and how slippery it has become to pin down. Meta has expanded far beyond Facebook’s roots as a bulletin board for college students, and scores of newer companies have developed similar products, emulating popular features such as the “like” button and news feed.

In the first four weeks of the trial, a parade of social media executives from companies including Reddit, Pinterest and LinkedIn have done little to help clarify a social networking definition. They acknowledged that they all competed for the same users, but in many cases offered very different products.

Defining where Meta fits into the social media landscape will be the first and most important decision for Judge James E. Boasberg of the U.S. District Court for the District of Columbia, who is presiding over the trial.

It will be “no walk in the park,” Judge Boasberg wrote in an opinion late last year.

The case examines whether Meta’s purchases of Instagram in 2012 for $1 billion and WhatsApp in 2014 for $19 billion illegally quashed competition. Judge Boasberg’s decision will have broad implications for the tech market as the industry faces a yearslong bipartisan push to curb Silicon Valley’s power and grip over speech, entertainment, commerce and computing.

If he sides with the government, which has said it seeks to break up Meta, the decision could deter the voracious appetites of the biggest tech companies to buy smaller rivals. That would shake up the start-up economy, where many founders rely on bigger players to acquire their companies for huge sums of money, allowing investors to cash out.

“It is a significant case because the world we’re in now has gotten a lot more complex, and so if the F.T.C. wins, there will likely be more aggressive antitrust enforcement,” said Daniel Rubinfeld, a former deputy assistant attorney general at the Justice Department who worked on the government’s antitrust case against Microsoft more than two decades ago.

In most antitrust cases, the competitive market is easier to define, legal experts said. Prices are used as the foundation to evaluate a company’s power and effect on competition. That could include a merger or anticompetitive behavior that pushes up prices for airline tickets or home appliances, for example.

But internet companies like Meta offer free services to consumers, turning its case into a novel legal debate.

In his opening statements, Daniel Matheson, the government’s lead lawyer in the case, accused Meta of being “a monopolist of personal social networking services in the United States,” with two competitors: Snap and the tiny app MeWe.

Mr. Matheson argued that Meta’s network of people who knew one another was key to the company’s growth and that it attracted advertisers that were interested in users promoting goods to their close connections.

Meta fired back, saying that it now primarily competed for the attention of users who scrolled through short-form videos on YouTube and TikTok. Its top litigator, Mark Hansen, said the company went into “crisis” mode when TikTok became available in the United States in 2018.

On Thursday, one of Meta’s lawyers asked Adam Mosseri, the head of Instagram, if the app was more like Facebook or TikTok.

“I’d put Instagram between the two, but much closer to TikTok,” he said. Instagram began as an app to connect friends, he added, but users now turn to it much more for entertainment.

Clouding the picture, the parade of executives from other social media companies have done little to define the industry’s market.

“YouTube and Instagram are TikTok’s most important competitors,” according to an internal TikTok document from 2021 presented by Meta’s lawyers.

When asked about the rivalry, Adam Presser, TikTok’s head of operations, undercut the idea by saying the apps function differently: “I don’t think of us as a social app.”

YouTube is mainly used for entertainment, and people rarely use the platform to share content or follow other users they know, said Aaron Filner, a senior director at the company.

When it comes to the social media site X, “I would guess that more people these days think of it as a place to see what’s new and what’s happening in the world versus thinking of it as a place to share pictures and whatnot with friends and family,” said Keith Coleman, the company’s vice president of product.

Legal experts said it was typical to squabble over market definitions.

In 1997, the F.T.C. successfully sued to block a merger of Staples and Office Depot, warning of concentration in the office supply store market. The companies had argued they competed against other retailers like Walmart.

The next year, the government accused Microsoft of squeezing competition by tying its internet browser to its popular Windows operating system. The government persuaded the judge to narrowly define the market in the case as personal computers that run on Intel chips, excluding Apple computers and hand-held devices.

“The F.T.C. in the Meta case is taking a traditional approach to defining markets narrowly, but the challenge here is that the market feels different because it is digital and it makes sense that the competition is for eyeballs and attention,” said John Newman, a professor of law at the University of Miami and a former F.T.C. official who worked on the agency’s case against Meta.

Judge Boasberg has given little indication of his thinking. Still, he has noted that the various social media apps seem to have many of the same features, asking if the way they are used is “just a difference in degree.”

He noted that texting had supplanted voice calls, something he described as “elderly communications.” Younger users are even more facile in switching up platforms and technologies.

“Aren’t those norms changing all the time?” Judge Boasberg, who doesn’t use social media, asked an expert witness.

Several weeks after a federal appeals court said Apple would have to loosen its grip on its App Store, Tim Cook, the company’s chief executive, and his top lieutenants debated what to do.

For more than a decade, Apple had required apps to use the App Store payment system and collected a 30 percent commission on app sales. Now, in 2023, the courts were ordering it to allow apps to avoid Apple’s payments and go directly to online consumers. Mr. Cook wanted to know: Could Apple still charge a commission on those sales without violating a court order?

Phil Schiller, who oversaw the App Store, worried that new fees could be illegal. He favored making online sales free of an Apple commission. Luca Maestri, who oversaw the company’s finances, disagreed. He favored charging a commission of 27 percent for online sales because it would protect the company’s business.

Mr. Cook sided with Mr. Maestri, and Apple set out to justify that choice. It “manufactured” an independent economic study to legitimize its decision, a federal judge said in an angry ruling last week. It withheld thousands of documents under attorney-client privilege claims. And at least one of its executives lied on the witness stand.

The judge’s ruling, as well as witness testimony this year and company documents released on Thursday, shows the extraordinary measures that Apple took to keep every penny it collected in the App Store. The decision by Judge Yvonne Gonzalez Rogers, who heard the initial lawsuit brought by the video game company Epic Games in 2020, could cast a shadow over Apple’s business for years, weakening its credibility as legal scrutiny of its operations intensifies.

The company is also trying to fend off a half dozen other legal challenges, including a Justice Department antitrust lawsuit accusing it of maintaining an iPhone monopoly, class action lawsuits from app developers in the United States and anticompetitive investigations of its App Store by the European Union, Britain, Spain and potentially China.

“If you burn your credibility with the courts, the next judge is going to be a lot less willing to forgive,” said Mark A. Lemley, a Stanford University professor of antitrust and technology law. In future cases for Apple, he said, “it’s going to be easier for a judge to jump to the conclusion that people are lying.”

Google has shown that a company’s actions can cast a shadow over high-stakes legal proceedings. Last month, in an antitrust case over its advertising technology, a judge said the company’s efforts to conceal its communications had raised questions about whether it would follow the court’s remedies for its behavior.

Apple is appealing Judge Gonzalez Rogers’s ruling, which held the company in civil contempt. In requesting a delay of the court’s order to loosen its grip on the App Store, Apple said on Wednesday that it would show the contempt finding was “unwarranted.” The company declined to comment further for this article.

Epic, the developer of Fortnite, sued Apple in 2020, accusing it of violating antitrust laws by forcing developers to use its App Store payment system. Judge Gonzalez Rogers ruled largely in favor of Apple, finding it wasn’t a monopoly, as Epic had argued. But she said Apple had violated California competition law and ordered the company to allow apps to include links and buttons to buy software and services outside the App Store.

Apple created a task force, code-named Project Wisconsin, to respond to the order. It considered two different solutions. The first would allow apps to include links for online purchases in restricted locations, free of a commission. The second would allow apps to offer those links where they wished but force them to pay a 27 percent commission on sales.

With links and no commission, Apple estimated it could lose hundreds of millions of dollars, even more than $1 billion. With a 27 percent commission, it would lose almost nothing.

Mr. Cook met with the team in June 2023. He reviewed a range of commission options, from 20 to 27 percent. He also evaluated analysis showing that few developers would leave Apple’s payment system for their own if there was a 27 percent commission, court records show. Eventually, he chose that rate while also approving a plan to restrict where apps put links for online purchases.

Afterward, Apple hired an economic consultant, Analysis Group, to write a report that Apple could use to justify its fees. The report concluded that Apple’s developer tools and distribution services were worth more than 30 percent of an app’s revenue.

Apple also created screens to discourage online purchases by making them seem scary and “dangerous,” court documents show. Mr. Cook weighed in, asking the team to revise a warning to emphasize Apple’s privacy and security. Rather than “You will no longer be transacting with Apple,” the company said: “Apple is not responsible for the privacy or security of purchases made on the web.”

When Apple revealed its 27 percent commission in January 2024, Epic filed a claim in court that Apple wasn’t complying with the judge’s order. Judge Gonzalez Rogers brought Apple and Epic back to court. Alex Roman, a vice president of finance, testified that Apple had made its final decision on its commission on Jan. 16, 2024. Executives also testified that the Analysis Group report had helped them set the commission rate.

Judge Gonzalez Rogers questioned whether Apple was telling the truth and asked the company to provide documents about its plans. It produced 89,000 documents but claimed a third of them were confidential. The court said those claims were “unsubstantiated” and forced Apple to turn over more than half of the documents.

The documents made clear that Mr. Roman had lied under oath, that the Analysis Group report was a “sham” and that Apple had “willfully” disregarded a court order, Judge Gonzalez Rogers said. She called it a “cover-up.”

Her ruling will give prosecutors, regulators and judges ammunition against Apple’s defense strategies in a half dozen similar cases around the world, several antitrust and tech law professors and lawyers said.

When the company tries to redact or withhold documents, prosecutors and judges can point to how those strategies were found to be “tactics to delay the proceedings” in the Epic Games case, these experts said. When Apple executives testify, prosecutors and judges could question their credibility because the company was found to “hide the truth” and “outright lie.”

In the Justice Department’s antitrust case and others against Apple, said Colin Kass, an antitrust lawyer at Proskauer Rose, courts and regulators seeking Apple documents “will start the process by saying, ‘Open your doors, and don’t you dare try those silly little games you used in the past.’”

The company will face more skepticism about defenses, as well, in the Justice Department’s lawsuit, said Rebecca Haw Allensworth, a law professor at Vanderbilt University who studies antitrust. In the past, Apple has said it shows green bubbles for an Android owner’s messages because communicating across smartphone systems is less secure. But she said those claims might be considered less credible after the Epic ruling.

Ms. Allensworth said the judge’s opinion also could stiffen the resolve of the European Union, Britain, Spain and others pressing Apple to change its App Store practices because regulators and courts often find safety in numbers.

“Apple has been acting like they’re above the law,” she said. “This sends a signal Apple is not.”

On a frigid day in early January, as she worked in her office in the Boston suburb of Billerica, Mass., Siyu Huang received a two-word text message.

“Spinning wheels,” it said. Attached was a short video clip showing a car on rollers in an indoor testing center.

To the untrained eye there was nothing remarkable in the video. The car could have been getting its emissions tested at a Connecticut auto repair shop (except it had no tailpipe). But to Ms. Huang, the chief executive of Factorial Energy, the video was a milestone in a quest that had already occupied a decade of her life.

Ms. Huang, her husband, Alex Yu, and their employees at Factorial had been working on a new kind of electric vehicle battery, known as solid state, that could turn the auto industry on its head in a few years — if a daunting number of technical challenges could be overcome.

For Ms. Huang and her company, the battery had the potential to change the way consumers think about electric vehicles, give the United States and Europe a leg up on China, and help save the planet.

Factorial is one of dozens of companies trying to invent batteries that can charge faster, go farther, and make electric cars cheaper and more convenient than gasoline vehicles. Transportation is the biggest source of man-made greenhouse gases, and electric vehicles could be a potent weapon against climate change and urban air pollution.

The video that landed in Ms. Huang’s phone was from Uwe Keller, the head of battery development at Mercedes-Benz, which had been supporting Factorial’s research with money and expertise.

The short clip, of a Mercedes sedan at a research lab near Stuttgart, Germany, signaled that the company had installed Factorial’s battery in a car — and that it could actually make the wheels move.

The test was an important step forward in a journey that had begun while Ms. Huang and Mr. Yu were still graduate students at Cornell University. Until then, all their work had been in laboratories. Ms. Huang was excited that their invention was venturing into the world.

But there was still a long way to go. The Mercedes with a Factorial battery hadn’t yet been taken out on the road. That was the only place the technology really mattered.

Many start-ups have produced solid-state battery prototypes. But no American or European carmaker has put one into a production vehicle and proved that the technology could survive the bumps, vibrations and moisture of the streets. Or if any have, they have kept it a secret.

In late 2023, Mr. Keller, a veteran Mercedes engineer, proposed to Ms. Huang that they try.

“We’re car guys,” Mr. Keller said later. “We believe in things really moving.”

Roots in China

Ms. Huang stands out in a niche dominated by men from Silicon Valley. Some brag about their 100-hour workweeks; she believes in a good night’s sleep. “Having a clear mind to make the right decision is more important than how many hours you work,” she said.

She is approachable and laughs easily, but also projects determination. She works from a sparsely decorated office in Billerica that looks out on a patch of forest crossed by power lines. The furnishings include a plain black bookcase, stocked with a few technical volumes, that she inherited from a previous tenant. Her diplomas from Cornell — a Ph.D. in chemistry and a master’s in business administration — hang on the wall.

Ms. Huang grew up in Nanjing, China, where she was in an elementary school program that had her gather environmental data. The program instilled an interest in chemistry and an awareness of the vehicle exhaust and industrial pollution choking Nanjing’s air. She realized, she recalled, that “we need to grow a planet that’s healthier for human beings.”

In a dormitory at Xiamen University on China’s southern coast, where she studied chemistry, she saw an advertisement for a Swedish exchange program. After spending two years there, she and Alex, whom she had known since they were students in China, were both accepted to doctoral programs in Cornell’s chemistry department. She arrived in Ithaca, N.Y., in 2009 with $3,000, which she had managed to save from her Swedish scholarship. They have both since become U.S. citizens.

They were star students, said Héctor Abruña, a professor at Cornell known for his research in electrochemistry. He still has a picture on his office bookshelf of himself with Mr. Yu and Ms. Huang in their commencement robes.

With an idea that grew out of Dr. Abruña’s lab and some seed money from the State of New York, Mr. Yu and Ms. Huang founded the company that later became Factorial while she was still completing her business degree.

“They are extremely dedicated and extremely bright,” said Dr. Abruña, who continues to advise Factorial. “Straight shooters — zero BS.”

Mr. Yu is now Factorial’s chief technology officer. The company is, in that sense, a family operation. Ms. Huang is reticent about their private life, declining to say even how many children they have.

Initially the company focused on improving the materials that allow batteries to store energy. That changed after Mercedes invested in Factorial in 2021. Mercedes was looking for a bigger technological leap and encouraged Factorial to pursue solid state.

The technology has that name because it eliminates the liquid chemical mixture, known as an electrolyte, that helps transport energy-laden ions inside a battery. Liquid electrolytes are highly flammable. Replacing them with a solid or gelatinlike electrolyte makes batteries safer.

A battery that doesn’t overheat can be charged faster, perhaps in as little time as it takes to fill a car with gasoline. And solid-state batteries pack more energy into a smaller space, reducing weight and increasing range.

But solid-state batteries have one big drawback that explains why you can’t buy a car with one today. Such battery cells are more prone to grow spiky irregularities that cause short circuits. Vast riches await any company that can overcome this problem and develop a battery that is durable, safe and reasonably easy to manufacture.

Despite obvious differences between Factorial and Mercedes — the start-up has a little more than 100 employees, compared with 175,000 — Ms. Huang’s working style meshed with the culture at Mercedes and its roots in Swabia, the region around Stuttgart where people are known for their no-nonsense approach and restraint.

Mr. Keller found Ms. Huang’s low-key, factual manner to be a welcome contrast to the hype and unfulfilled promises that are pervasive in the battery and technology industries. Factorial, he said, “has not been announcing, announcing, announcing and not delivering.”

‘Production hell’

It’s an axiom in the battery business that producing a cool prototype is the easy part. The challenge is figuring out how to make millions of solid-state batteries at a reasonable price.

Factorial confronted that problem in 2022, setting up a small pilot factory in Cheonan, South Korea, a city near Seoul known for its tech industry. The project became, in Ms. Huang’s words, “production hell” — the same phrase Elon Musk used when Tesla was struggling to mass-produce a sedan and nearly went bankrupt.

To make money, a battery factory can’t produce too many defective cells. Ideally the yield, the percentage of usable cells, should be at least 95 percent. Hitting that target is devilishly difficult, involving volatile chemicals and fragile separators layered and packaged into cells with zero margin for error. The machinery doing all this is encased in Plexiglas chambers and overseen by workers dressed in head-to-toe protective gear to prevent contamination.

Dozens of companies are trying to mass-produce solid-state cells, including big carmakers like Toyota and smaller ones like QuantumScape, a Silicon Valley start-up backed by Volkswagen. Mercedes, hedging its bets, is also working with ProLogium, a Taiwanese company.

Nio, a Chinese carmaker, sells a vehicle with what it advertises as a solid-state battery. Analysts say the technology is less advanced than what Factorial is developing, offering fewer advantages in weight and performance. But there is little doubt that Chinese companies are investing heavily in solid state. Nio did not respond to a request for comment.

Every company has its own closely guarded recipes and manufacturing processes. “It’s difficult to say which technology will win,” said Xiaoxi He, a technology analyst at IDTechEx, a research firm.

Partly because solid-state batteries are so difficult to manufacture, many auto executives are skeptical that they will make commercial sense anytime soon. Shares in many solid-state battery start-ups have plunged, and management turmoil is common.

Factorial has insulated itself from the harsh judgments of Wall Street by never selling stock. Its funding comes from private investors including WAVE Equity Partners, a Boston firm, and partners that include the South Korean automaker Hyundai; LG Chem, a South Korean company that makes battery materials; and Stellantis, which next year plans to test Factorial batteries in Dodge Charger muscle cars.

Projections of how soon solid-state batteries would be available have proved overly optimistic. Toyota displayed a futuristic prototype in 2020, but the company is still years away from selling a car with a solid-state battery.

Kurt Kelty, a vice president at General Motors in charge of batteries, is among those who will believe it when they see it. “We’re not banking on solid state,” Mr. Kelty said.

In the beginning, Factorial’s prototype assembly line in South Korea had a yield of just 10 percent, meaning 90 percent of its batteries were faulty. Despite her preference for a good night’s sleep, Ms. Huang often had to wake up at 4 a.m. to deal with problems at the factory, which was operating around the clock. She was in South Korea at least once a month.

“There were always issues,” she said. “There was a point, I was like, I don’t even know if we can make it.”

By 2023, Factorial had produced enough cells suitable for an automobile that Mr. Keller, a soft-spoken, amiable man who has worked at Mercedes for 25 years, began thinking about installing them in a car. The cost and the risk of failure were high enough that he sought approval from his bosses. Armed with PowerPoint slides, Mr. Keller went to Ola Källenius, an imposing Swede who is chief executive at Mercedes.

Mr. Källenius’s office is at the top of a glass and steel high-rise in the middle of a sprawling manufacturing and development complex beside the Neckar River in Stuttgart.

Mr. Keller argued that road testing would help determine, among other things, whether the batteries would work with air cooling alone. If so, that would eliminate the need for a heavier, more costly liquid-cooled system.

Mr. Källenius signed off on the project, reasoning that a tangible goal would motivate the team and hasten development. He drew an analogy to Formula 1 racing. “If you’re chasing the leader, and suddenly you can see him, you get faster,” Mr. Källenius recalled.

Ms. Huang was a bit surprised when, in late 2023, Mr. Keller told her that Mercedes wanted to put the cells in a working vehicle. “We didn’t realize it was coming so soon, honestly speaking,” she said with a laugh.

But by June 2024, Factorial had managed to produce enough high-quality cells to announce that it had begun delivering them to Mercedes. In November, the factory in South Korea hit 85 percent yield, the best result yet. Ms. Huang and the Korean team celebrated by going out to a barbecue joint.

Mercedes still had to figure out how to package the cells in a way that would protect them from highway dirt and moisture. And it had to integrate the battery pack into a vehicle, connecting it to the car’s control systems.

The Factorial cells had one big drawback that made them hard to install in a car. They expanded when charged and shrank when discharged. In Mr. Keller’s words, they “breathed.”

Mr. Keller turned to engineers on the Mercedes Formula 1 racing team, who are accustomed to quickly solving technical problems. They devised a mechanism that expanded and shrank with the cells, maintaining constant pressure.

By Christmas 2024, a team working at Mercedes’s main research center in Sindelfingen, outside Stuttgart, texted Mr. Keller those two words: “spinning wheels.”

‘Finally I see you’

Mr. Keller confessed that he got a little emotional when his team sent him the video of the car. He waited until after Christmas to forward it to Ms. Huang with the same two words.

Several weeks later, the Mercedes engineers took the car with Factorial’s battery, an otherwise standard EQS electric sedan, to a company track for its first road test.

The engineers drove the car slowly at first. They carefully monitored technical data displayed on the dashboard screen.

They drove faster and faster until, by the fourth day, they reached autobahn speeds of 100 miles per hour. The battery didn’t blow up. In theory, it can power the car for 600 miles, more than most conventional cars can travel on a tank of gasoline.

Mr. Keller had been keeping Ms. Huang apprised of the progress, but she was still surprised when, during a meeting on marketing strategy in February, people from the Mercedes communications department mentioned that they had written a news release announcing the achievement.

“Do you want to take a look?” they asked.

She certainly did. The first successful road test with a Factorial battery was an enormously important moment, one they had been anticipating for years. Yet the teams at Mercedes and Factorial did not throw parties to celebrate. They still had work to do.

The next step is to equip a fleet of Mercedes vehicles with batteries, perfect the manufacturing process and do the testing required to begin selling them. That will probably take until 2028, at least. Many experts don’t expect cars with solid-state batteries to be widely available until 2030, at the earliest.

In April, Ms. Huang finally found time to travel to Stuttgart and ride in the car herself.

It was a clear spring day, with greenery sprouting in the German countryside and flowers beginning to bloom. Mercedes employees escorted her to a garage in Sindelfingen, where the automaker also has a large factory complex.

Ms. Huang had seen many photos of the car, but she still felt a thrill when the garage doors opened. It felt “like a long-lost friend,” she said. “Like, ‘Finally I see you!’”

A Mercedes driver took her for a spin on the test track, zooming down an asphalt straightaway then around a banked curve that, Ms. Huang said, felt like a roller coaster.

Inside the car, there was no way to perceive the difference with the Factorial battery compared with a conventional one. “But it’s just so special because it’s with our battery.”