William Fitzhugh

A battery that heals itself, looking for a factory

Most lithium-metal batteries fail the same way. Tiny spikes of metal called dendrites grow through the cell, short it out, and sometimes set it on fire. The battery world spent decades treating lithium metal as the energy-dense prize you could never safely touch. William Fitzhugh's answer was to build an electrolyte that lets the spikes form and then squeezes them back into line - a cell that, in effect, repairs its own cracks. Adden Energy calls it self-healing. The marketing team likes the word. The physics earned it.

That single idea is why he runs a company instead of a lab. Adden Energy is the Harvard spin-out he co-founded to drag a thumbnail-sized coin cell across the widest chasm in hardware: the gap between a result that works on a bench and a product that works on an assembly line ten thousand times a day. The current job is unglamorous and enormous - a roll-to-roll pilot line at the company's Waltham, Massachusetts headquarters, the kind of machinery that turns a chemistry into a manufacturable thing.

He talks about the destination in pump-station terms. Not kilowatt-hours, not energy density curves - the gas station. The promise he keeps repeating is a car you can refill in the time it takes to refill a tank of gasoline, that goes twice as far, and lasts close to ten times longer before it wears out. It is a deliberately ordinary ambition dressed in extraordinary numbers.

The argument starts with a driveway, not a battery

Ask most battery founders why their cell matters and you get a lecture on energy density. Fitzhugh starts somewhere stranger: the American garage, or the lack of one. Roughly 37 percent of Americans, he points out, don't have a garage at home. No garage means no slow, cheap overnight charge. No overnight charge means the EV pitch quietly excludes a third of the country.

That reframe is the whole thesis. If you can't charge slowly at home, the battery has to charge fast somewhere else - fast enough that a public charger feels like a gas stop, not a lunch break. Performance, in his telling, isn't about bragging rights. It's about who gets to participate. The faster cell isn't a luxury feature. It's the price of admission for everyone without a driveway.

Underneath sits the bigger lever he keeps coming back to. He calls full electrification of the vehicle fleet one of the most significant steps we can take to combat climate change. The battery is a means. The fleet is the point.

What the lab cell actually did

Benchmarks reported from Adden Energy's lab-scale cells. These are coin-cell results - the reason a pilot line exists is to find out how much of this survives scale.

From a Harvard lab bench to a license agreement

The technology wasn't invented in a garage. It was invented in the lab of Xin Li, an associate professor of materials science at Harvard's School of Engineering and Applied Sciences, where Fitzhugh did the doctoral work that earned him a PhD in applied physics in 2020. Fellow graduate student Luhan Ye worked alongside him on the design. When the science looked like it might leave the bench, Harvard's Office of Technology Development granted an exclusive license, and the four founders - Fitzhugh, Li, Ye, and investor Fred Hu of Primavera Capital - turned a paper into a company.

The transition from physicist to CEO is the part founders rarely romanticize, and Fitzhugh is blunt about what made it work. His advice to other technical founders is almost anti-heroic: surround the science with people who speak the language of industry. Working with business-oriented people, he has said, was crucial for interacting with the bureaucracy of legacy OEMs - the automakers whose purchasing departments can outlast any startup's patience. The breakthrough gets the headline. The org chart gets the deal.

$5.15M to start, $15M to scale

Adden Energy launched publicly in 2022 with that Harvard license and a $5.15M seed round led by Primavera Capital Group, with Rhapsody Venture Partners and MassVentures along for the ride. The seed money bought the right to ask a harder question: does the chemistry hold up when you make the cells bigger?

In October 2024 the answer came back interesting enough to draw a $15M Series A led by At One Ventures, a fund built specifically around technologies that try to make industry kinder to the planet. The round pushed total funding to roughly $25M and was earmarked for the thing that turns a science project into a supplier: a roll-to-roll pilot production facility in Waltham. It is the least glamorous line item in the press release and the most important one in the building.

Four things that explain him

On the record

The valley of death, with a roadmap

Every hardware founder eventually meets the same wall: the result that dazzles at coin-cell size and disappoints at commercial size. Fitzhugh's entire 2024-2025 has been spent walking straight at it - building the pilot line, showing up at Battery Tech USA and battery safety summits, talking to the automakers whose timelines move in years, not quarters. Before Adden, he founded a company called American Nanotechnologies and passed through the Innovation Crossroads program, so the long-horizon grind of hard tech isn't new to him.

The bet is unresolved by design. If the self-healing cell holds its numbers at scale, the payoff is a battery that charges like a gas fill-up and reaches the third of the country without a garage. If it doesn't, it joins the long list of beautiful lab results that never found a factory. Either way, the next chapter gets written in Waltham, on a machine that has to do the same thing ten thousand times without flinching.

Why lithium metal scared everyone, and what changed

To understand why a three-minute charge is hard, start with the trade everyone in the field knows. Lithium metal holds far more energy than the graphite used in ordinary lithium-ion batteries. Pack that energy in and you double the range. The catch is that lithium metal is unruly: charge it fast and it grows those branching filaments, the dendrites, that worm through the cell, pierce the separator, and short it out. Decades of battery research treated this as a near-law of nature. You could have the energy density or the safety, not both.

Adden Energy's design reorders that compromise. Rather than trying to stop dendrites from ever forming, the solid electrolyte is engineered to control where they go and to push them back - the behavior the company markets as self-healing. The practical consequence is a cell that tolerates the brutal, fast charge currents that make dendrites worse in conventional designs. That tolerance is what turns "three minutes" from a marketing line into a measured result, and it is why the same cell can be cycled past ten thousand times instead of degrading in the hundreds.

None of this matters, of course, until it can be built by the meter rather than the millimeter. A coin cell is hand-assembled. A roll-to-roll line coats and stacks battery materials continuously, like a printing press for electrodes. Crossing from one to the other is where promising chemistries usually die, defeated by defects that don't show up at small scale. That crossing is the entire purpose of the Waltham facility, and the reason a physicist now spends his days thinking about coating tolerances and yield.

A founder who has done the long version before

Fitzhugh did not arrive at deep tech by accident. He started a company, American Nanotechnologies, in 2015, years before the Harvard battery work matured, and passed through Innovation Crossroads, the entrepreneurial fellowship tied to national-lab research. By the time the Adden Energy science was ready to leave the bench, he had already lived the slow, capital-hungry rhythm of building hardware - the kind of timeline that scares off founders trained on software's fast feedback loops.

That patience shows up in how he runs the company. The team is small, around two dozen people, deliberately weighted toward the unglamorous work of manufacturing rather than the demo-day theatrics. The backers he chose - At One Ventures, Primavera, Rhapsody, MassVentures - are the kind that underwrite years, not quarters. And his public posture stays consistent: show up at the battery and safety conferences, talk to the automakers in their own bureaucratic dialect, and resist overpromising on numbers that still have to clear the factory floor.