Asked. And answered.

Each card has a one-sentence TL;DR up top, a plain-English paragraph in the middle, and an engineering-case block at the bottom with numbers, citations, and trade-offs. You can stop at any layer. The plain-English layer is written for a curious non-specialist; the engineering layer is written for a reviewer with a chemistry or finance background. If a card spans more than one screen, that’s on purpose — the depth is there if you want it.

This FAQ is the public-facing decision document. It does not cover: detailed cell-level schematics (those live in the Operations Manual at 11_operations.html); MOFCOM/CFIUS legal opinions on Chinese collaboration (handled by Stage 0 trade counsel); customer NDAs and named pilot agreements (under confidentiality until contracts close); the full 30-year service schedule (Operations Manual); the proposal document itself (01_proposal.html). If you’re looking for a specific topic and don’t find it here, check the rest of the hub at saltwaterpower.org.

Reviewers should know what this FAQ is honestly uncertain about. Three theses remain open: (1) whether Chen’s coin-cell chemistry transfers to a 3 MWh skid at C/4 grid rate (Phase 1 directly tests this); (2) whether 30-year calendar aging holds up under the electrolyte-drift, seal-degradation, and CO₂-ingress envelope (accelerated stress tests + Year-10/20 service intervals address this); (3) whether hyperscaler procurement actually prices $/kWh-cycle over $/kWh upfront (Stage 0 monitors RFP language; the eight-surface revenue structure holds even if the carbon-credit surface goes to zero). The card “What evidence would change your mind and kill the program?” in Section F names the five specific kill-criteria.

It stores grid electricity. The chemistry uses salt water (specifically magnesium chloride) instead of the flammable lithium electrolytes most batteries use, which is why it can’t catch fire. The two electrodes are made of materials that let magnesium ions move in and out repeatedly without breaking down, which is why it lasts so much longer than a lithium battery.

Two electrodes (anode and cathode) sit on either side of an electrolyte that lets ions pass but not electrons. Charging pushes ions and electrons apart; discharging lets them recombine, and the electrons take the long way around through whatever you’ve plugged in. The chemistry of the electrodes determines how much energy you can store, how fast you can move it, and how many cycles you can repeat before things wear out.

A research team at City University of Hong Kong combined three components that nobody had combined before: a neutral-pH salt-water electrolyte, a polymer anode that stores charge through radical chemistry rather than physical insertion, and a copper-doped Prussian blue cathode. The result ran longer than any aqueous battery on record — far past the point where lithium cells would have failed — without catching fire, leaking, or losing meaningful capacity.

Lithium-ion batteries use organic solvents that ignite at high temperatures — that’s the source of every laptop and EV battery fire. Water doesn’t burn. Aqueous batteries trade some energy density for a fundamentally non-flammable safety profile, which is exactly the trade you want for grid storage that doesn’t move and sits next to expensive infrastructure or people.

Lithium battery fires happen when an internal short heats the cell, the organic solvent decomposes and ignites, that releases more heat, more solvent decomposes, and so on until 800°C+ flame. With water as the electrolyte, there’s no fuel for that chain. The worst case is an internal short that produces some hot water, some hydrogen and oxygen gas in concentrations too dilute to burn, and the cell stops working — no flame, no thermal runaway, no propagation to neighboring cells.

The salt in this battery is the same salt that’s in the ocean, in your bag of road de-icer, and in the tofu coagulator at any Asian grocery. It’s rated GRAS (Generally Recognized As Safe) by the FDA. Spilling the electrolyte on a parking lot is comparable to dumping pickle brine: a cleanup nuisance, not a hazmat event.

A skid is a sealed system designed with a containment bund underneath. A puncture causes the affected module to disconnect and the electrolyte to drain into the bund where it can be pumped back into service or disposed of. The neighboring modules keep running because they’re electrically isolated. This is the opposite of lithium fires that propagate cell-to-cell across an entire container.

A reviewer who has watched battery startups die will read this proposal and think: aqueous, stationary-only, low energy density, non-battery-native company, $/kWh-cycle pitch. That is exactly Aquion’s structure. We don’t win this argument by saying “different chemistry” — the chemistry differed (Na-ion vs Mg-ion), but the failure pattern transfers. Honest comparison: Aquion raised $190M and went Chapter 11 in March 2017 because LFP’s cost-down curve outpaced their own scale-up. Their cells worked fine; their unit economics didn’t. Three differences mitigate (not eliminate) this proposal’s exposure: (1) Phase 1 validates chemistry for $1.4–1.8M before any major capital commits — vs Aquion deploying $50M+ before their first commercial cell; (2) Plantweb subscription is a structurally different revenue surface that Aquion never had access to and LFP integrators still don’t; (3) projected 8,000–25,000 cycles at C/4 (rate-derated, see Chemistry section) versus Aquion’s ~3,000-cycle ceiling. The chemistry-difference defense is misdirection; the stage-gating and subscription-moat differences are the actual answer.

Both teams used organic solvents (like the ones in lithium batteries) with magnesium salts dissolved in them. The problem: magnesium deposits in organic solvents form a passivation layer on the anode — a kind of chemical scab that blocks further ion movement. Pellion (an MIT spinout) gave up and pivoted to lithium-sulfur, then shut down. Toyota’s program remains in the lab after more than 15 years. Chen et al.’s breakthrough was moving Mg-ion into aqueous neutral pH, where this passivation chemistry physically cannot happen.

All three components had been studied separately in other contexts. PBA cathodes were used in sodium-ion batteries (Natron Energy is scaling those now). COP polymers were explored as organic electrodes for various chemistries. Aqueous Mg-ion was attempted at low voltage. But this specific three-part recipe — CuFe-PBA + Hex-TADD-COP + neutral-pH MgCl₂ — was new. That’s what makes the Feb 2026 result legitimately first-of-kind, not a refinement of something existing.

There is one other Feb 2026 paper from a Chinese university group showing 75,000 cycles in aqueous magnesium-ion using a different anode chemistry (Ta-doped MoO₃). That confirms aqueous Mg-ion can run ultra-long-life cycles in principle, but doesn’t replicate Chen’s specific combination. Phase 1 of this proposal is exactly the work to become the second lab in the world that has validated Chen’s cell at grid-relevant rates.

Cycle life is theoretical math. In practice, the cells themselves can run far longer than the surrounding equipment (frame, plumbing, electronics, electrolyte seals), so the design target is permanent infrastructure on a 15–30 year horizon with periodic service. That’s 2–3× the life of a typical lithium grid skid, which gets fully replaced every 8–10 years.

The published paper used extreme charge rates for accelerated testing. Grid storage doesn’t need that — it needs to dispatch over 4 to 8 hours. The open question Phase 1 answers: does the 120,000-cycle life still hold when the cells are run at gentler 4-hour discharge rates the way they’d actually be deployed? Industry expectation is yes (slower is usually easier on a cell, not harder), but it must be measured.

Energy density doesn’t appear in the customer’s bill directly, but it shows up indirectly as footprint, slab tonnage, shipping mass, and electrolyte capex. A 3 MWh aqueous skid is roughly 2.5× the footprint and ~1.5× the slab cost of a Tesla Megapack at the same capacity. We don’t hide that delta; we beat it on $/kWh-cycle. Over 8,000–25,000 grid-rate cycles at 80% DoD, the LCOE math (see Section C) still shows a 3–8× advantage on lifetime delivered energy. The trade is real, the customer math is favorable, and the only application this chemistry is categorically wrong for is vehicles — which is why it’s stationary-only by design.

If you put 100 kWh in, you get 70–80 kWh back out — the rest is lost as heat. Lithium-iron-phosphate is similar. Iron-air loses about half because it has to convert chemicals through an oxygen-breathing reaction. For daily charge-discharge applications, those efficiency differences compound enormously over 30 years; for long-duration backup (where the battery sits charged for days waiting for a storm), efficiency matters less.

Every electron that enters a battery during charge should come back out during discharge. The ratio is CE, and it should approach 100%. But the first ~50 cycles always look messy because the cell is settling: solid-electrolyte interphase forming, electrolyte wetting the porous electrodes, hardware degassing. A blanket “CE ≥99.5% from cycle 1” rule would kill cells that recover to 99.95% by cycle 200 and then run forever. So Phase 1 tiers the threshold: ≥97% during formation (cycles 1–50), ≥99.0% during break-in (50–200), ≥99.5% in steady-state. That mirrors how cycler engineers actually evaluate cells.

Chen et al. ran 120,000 cycles on a 0.5 cm² coin cell at high current. That’s demonstrated. Phase 1 will run a few hundred cycles on bigger pouches at realistic C-rates — also demonstrated. The 120,000 number does not automatically transfer to a multi-MWh skid; geometry, current distribution, edge effects, and 30-year calendar aging all change the answer. The proposal cites both numbers, labels which is which, and treats the projection as a thesis to be tested — not a sales claim. Claim 7 in the proposal was retitled “Inherent Safety Now, Lowest-Carbon Bet for the Second Decade — a Testable Thesis” precisely to remove the implication that scale is guaranteed.

The sharpest reviewer question is: “you cite 120,000 cycles, but that was a coin cell at 20 amps per gram. How does it translate to a 3 MWh skid running at a 4-hour discharge rate?” Honest answer: it doesn’t translate directly. Polarization at higher current accelerates the chemistry’s natural fade mechanisms; at lower current (C/4 vs 20 A/g), polarization is much smaller, so calendar aging becomes the dominant fade mode instead of rate-driven mechanical stress. Tafel extrapolation of Chen’s published per-cycle fade slope, corrected for reduced polarization and increased calendar time, projects 8,000–25,000 cycles at C/4 and 80% retention — still 2–6× LFP’s 3,000–6,000 cycles at the same retention floor. The $/kWh-cycle economics in Section C use the LOWER bound (8,000), not the headline 120,000. The Phase 1 kill-gate measures C/4 data on pouches and answers which end of the projection is closer to truth.

Lithium batteries fail catastrophically when the anode grows microscopic metal “dendrites” — tree-like spikes that pierce the separator and short the cell, creating the internal heat that starts thermal runaway. The same problem plagues sodium and zinc batteries at lower energy density. In aqueous Mg-ion, this failure mode is thermodynamically impossible: at the operating voltage, hydrogen evolution from water reduction wins the competition against Mg-metal plating, so no Mg° ever forms at the anode. The COP anode itself is an organic radical-cation host, not a metal-deposition surface. The dendrite shorting pathway that kills the lithium, sodium, and zinc families is structurally absent here.

A reviewer asking “is this really new?” deserves the honest map of prior work. Six bodies of literature converge at the cell Chen et al. published in February 2026: (1) Water-in-salt electrolyte — Suo, Borodin, Xu et al. Science 2015 established that extreme salt concentrations widen water’s electrochemical stability window; this is the foundational reference for any pH-7 high-concentration aqueous claim. (2) Multivalent Mg chemistry — Doron Aurbach’s group at Bar-Ilan University has been the canonical source on Mg-electrolyte interfaces since the 1990s; their work explains why non-aqueous Mg-metal anodes passivate and why aqueous Mg-ion sidesteps that failure mode. (3) Aqueous PBA cathodes — the Wang group at U Maryland and the Ji group at Oregon State established Prussian-Blue-analog cathodes in aqueous Zn / Na / K systems (~2017–2022); Natron Energy and Faradion scaled PBA Na-ion to commercial deployment. (4) Mg-ion in PBA frameworks — multiple academic groups (Whittingham at Binghamton, Cabana at Illinois Chicago) demonstrated Mg²⁺ intercalation in PBA at sub-2V before Chen’s 2.2V result. (5) Radical-polymer organic electrodes — the Schon, Tilley, and Nishide groups (Imperial College, Wisconsin, Waseda) built the prior art for TEMPO / nitroxyl / radical-cation polymer electrodes; the Hex-TADD-COP anode descends from that lineage. (6) Stationary-storage LCA — NREL ATB, PNNL Energy Storage Technology Center, EPRI BESS reports, and Argonne ANL benchmarks frame the cycle-life and lifetime-carbon math. Chen et al.’s contribution is the system-level integration of these six bodies into one cell that holds 120,000 cycles — not a discovery from zero.