Modulating the Lattice Stability in Sodium-Layered Oxide Cathodes through Inductive Effect

Abstract The stability and safety of sodium-layered oxide cathodes relative to lithium analogs remain a critical concern, yet their fundamental origin remains poorly understood. Here, we examine the role of sodium−oxygen bond ionicity in governing outgassing by introducing 5% lithium (Li), potassium (K), or both into layered NaNiO2 (NNO), yielding Na0.95Li0.05NiO2, Na0.95K0.05NiO2, and Na0.90Li0.05K0.05NiO2. Online electrochemical mass spectrometry at 4.3 V reveals that K substitution increases gas evolution with similar charge capacity, whereas Li substitution reduces gas evolution by nearly 80% with only a 16% capacity decrease. Capacity-controlled experiments show that NNO must be charged to only 3.8 V (144 mA h g−1) to achieve gas evolution comparable to Na0.95Li0.05NiO2 charged to 4.3 V (175 mA h g−1). These results demonstrate that tuning alkali-metal−oxygen bond ionicity through the inductive effect regulates Ni−O covalency, lattice oxygen stability, and interfacial reactivity, providing design guidance for safer sodium-layered oxide cathodes.