Engineering Voltage–Capacity Relationship for Predictable State-of-Charge in P2 Sodium-Ion Batteries

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Sodium-ion batteries (SIBs) are regarded as a promising alternative to lithium-ion technologies owing to the abundance of sodium resources and low costs. Beyond cathode degradation, a critical yet often overlooked challenge for practical applications is the poor predictability of state-of-charge (SOC) caused by non-linear voltage–capacity relationships. This issue is particularly severe in P2-type layered oxides, which are among the most competitive cathode candidates for high-energy SIBs. In P2-type layered oxides such as Na 0.67 Ni 0.33 Mn 0.67 O 2 , multiple Ni- and oxygen-related redox processes lead to several voltage plateaus, severely limiting voltage-based SOC estimation. We show that in the pristine material, a voltage deviation of only 0.03 V at 4.2 V can lead to a capacity estimation error of up to 34%, posing a substantial operational risk for battery management systems. Extensive literature surveys indicate that partial substitution of Ni with elements such as Zr, Li, Mg, and Cu can effectively increase the slope of the voltage–capacity profile. In particular, Cu can actively participates in reversible redox reactions, thereby redistributing the voltage plateaus. Motivated by these insights, Li and Cu were selected as representative dopants to elucidate the mechanism of voltage–capacity profile linearization through transition-metal substitution. Through targeted compositional modulation, the voltage plateaus are significantly shortened, yielding a near-linear voltage–capacity relationship. As a result, Li- and Cu-modified compositions reduce the SOC estimation error to below 3% under the same voltage uncertainty, without introducing additional system-level complexity, offering a materials-level design strategy for improving SOC reliability in SIBs. Acknowledgement This study’s subsequent research and related conference travel were funded by the Danish Independent Foundation (Grant Number: 5249-00027B).

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