A rechargeable cell without lithium

A rechargeable cell that uses none of lithium, cobalt, or nickel stores and returns as much energy, per gram of active material, as a lithium iron phosphate cell tested in the same hardware, and it does so at least as efficiently, both in charge recovered and in energy recovered.

Lithium iron phosphate cells are the cost and safety benchmark that a lithium-free cell has to meet if it is going to replace them in stationary storage. Cobalt and nickel are the other constrained metals in common cathodes. A half-cell against a lithium-metal counter electrode does not answer the question, and a cathode-only watt-hour figure leaves out the anode. The grade is a full cell, weighed, next to a lithium iron phosphate cell that had to work.

Voß and co-workers modeled large-format cells at a 90 μm cathode coating. In their hard-carbon-basic case the lithium iron phosphate pouch cell is 214 Wh kg⁻¹ and 507 Wh L⁻¹. Layered-oxide sodium-ion cells in that model span 147–206 Wh kg⁻¹ and 333–419 Wh L⁻¹. Polyanion sodium-ion cells span 158–192 Wh kg⁻¹ and 310–366 Wh L⁻¹. Prussian-blue-analogue cells span 161–185 Wh kg⁻¹ and 260–302 Wh L⁻¹. They write that several sodium cathodes, especially nickel-rich layered oxides, match or surpass lithium iron phosphate at the material level, and that the cell-level gap is largely the hard-carbon anode against graphite. Nickel-containing layered oxides are excluded by this problem even when the model puts them closest to the lithium iron phosphate pouch cell. The nickel-free classes in that model, polyanions and Prussian blue analogues, sit below it.

Those are modeled pouch and prismatic cells, not coin cells. This problem does not convert 214 Wh kg⁻¹ into a coin-cell pass mark. The grade is the cell built beside the candidate, because coin-cell watt-hours depend on loading, electrolyte fill, and formation. No absolute watt-hour target is cited.

Both cells are coin cells in identical cases, built in the same week and cycled on the same tester at 25 ± 2 °C. The control is a lithium iron phosphate cathode against a graphite anode. Its electrolyte and voltage window are those on the certificate of the lithium iron phosphate powder used. The candidate cell contains no lithium, cobalt, or nickel in the electrodes, the electrolyte, the binder, or the separator. Current collectors are aluminium, copper, or titanium. Every powder and salt is identified, and the absence of those three elements is taken from the supplier assay or from an elemental analysis of the electrodes.

Both cells are cycled at the same specific current: half the certificate specific capacity of that lithium iron phosphate, expressed as milliamperes per gram of cathode active mass, and applied to both cathodes. The candidate does not choose a gentler current by declaring a larger theoretical capacity. Each cell uses its own preregistered voltage window. The control uses the supplier’s window.

The anode mass is the mass of everything on the anode current collector that is not the current collector, weighed before assembly. A metal anode is weighed in full. Unreacted excess counts. The cathode active mass is the mass of the cathode storage material, excluding binder and conductive carbon, weighed before assembly. Those two masses are the denominator.

Cycling runs for 30 cycles. Cycles 1 through 4 are formation and are reported but not graded. Graded quantities are the averages, over cycles 5 through 30, of:

• specific energy, the discharge energy in watt-hours divided by the sum of the anode mass and the cathode active mass, as defined above
• Coulombic efficiency, discharge capacity divided by charge capacity
• round-trip energy efficiency, discharge energy divided by charge energy

Thirty cycles at this rate is about five days of cycling after assembly, so the grade fits in two weeks. The cycle count is a pass mark defined here, chosen for that reason. It is not a claim about calendar life or about a pouch cell.

Over cycles 5 through 30, the candidate’s average specific energy, average Coulombic efficiency, and average round-trip energy efficiency are each at least those of the control. Over the same cycles, the average discharge capacity per geometric cathode area is also at least the control’s. There is no absolute watt-hour target. The control measured beside the candidate is the target. Thirty cycles is the whole of the graded life. Cycle life beyond that window is outside this grade, because a longer test does not fit the two-week clock.

The control cell must itself behave like a working lithium iron phosphate cell: its discharge capacity on cycle 10 is at least 80% of the certificate specific capacity, and its average Coulombic efficiency over cycles 5 through 30 is at least 99%. Eighty percent and 99% are pass marks for the control, so that a weak control cannot hand the candidate an easy comparison. If the control fails those marks, the batch is rebuilt. It is not graded.

A cell that vents, leaks, or is removed from the tester fails. Capacity and energy are taken from the tester’s calibrated current, voltage, and time, not from a manufacturer’s advertisement. The same channel calibration is used for both cells.

Sodium, potassium, magnesium, calcium, zinc, aluminium, or organic electrode materials, including metal anodes of those elements other than lithium. Aqueous or non-aqueous electrolytes that obey the element rule. Any anode and cathode pair inside the element rule. The entire anode mass still counts.

Lithium salts, lithiated cathodes, cobalt, or nickel, including nickel foam, nickel tabs, and stainless-steel parts in contact with the electrolyte. Comparing a candidate half-cell that has a lithium-metal, sodium-metal, or other metal counter electrode against a full lithium iron phosphate cell, unless that metal electrode is the anode of a full cell and its whole mass is in the denominator. Quoting specific energy per gram of cathode only. Discarding cycles from the average. A primary cell. Treating formation cycles 1 through 4 as graded cycles. A cathode so thin that its areal discharge capacity falls below the control’s. Quoting cycle life beyond 30 cycles as part of this grade.

The candidate’s average specific energy over cycles 5 through 30 is at least half the control’s, and its average Coulombic efficiency and average round-trip energy efficiency are each at least the control’s. Half the control energy is a pass mark for this lesser bar. Matching the efficiencies without matching the energy is not a solution.

Pouch-cell figures of 214 Wh kg⁻¹ and 507 Wh L⁻¹ for lithium iron phosphate, and the sodium-ion ranges quoted above: Voß, Gruber, Mitterfellner, Plöpst, Degen, Schmuch, and Lux, Energy & Environmental Science, 2025, 18, 8104–8129, doi:10.1039/D5EE00415B (opens in a new tab). No coin-cell watt-hour number is cited, because the grade is the side-by-side cell. The formation exclusion, the 30-cycle length, the shared specific current, the mass definitions, the 80% and 99% control checks, and the partial-success ratio are pass marks defined here.

Open point: the Voß model is not a 30-cycle coin cell. A thin sodium-metal anode and a nickel-free cathode could still clear the active-mass comparison over this short window even though the modeled hard-carbon pouch cells do not. If an attempt clears the three averages on the rules written here, the honest revision is a longer graded window, not a denial of the data. This file does not invent a cycle life the model did not measure.