  Na-ion batteries (NIBs), which are recognized as a next-generation
alternative technology for energy storage, still suffer from commercialization
constraints due to the lack of low-cost, high-performance cathode materials.
Since our first discovery of Cu$^{3+}$/Cu$^{2+}$ electrochemistry in 2014,
numerous Cu-substituted/doped materials have been designed for NIBs. However
for almost ten years, the potential of Cu$^{3+}$/Cu$^{2+}$ electrochemistry has
been grossly underappreciated and normally regarded as a semielectrochemically
active redox. Here, we re-synthesized P2-Na$_{2/3}$[Cu$_{1/3}$Mn$_{2/3}$]O$_2$
and reinterpreted it as a high-voltage, cost-efficient, air-stable, long-life,
and high-rate cathode material for NIBs, which demonstrates a high operating
voltage of 3.7 V and a completely active Cu$^{3+}$/Cu$^{2+}$ redox reaction.
The 2.3 Ah cylindrical cells exhibit excellent cycling (93.1% capacity after
2000 cycles), high rate (97.2% capacity at 10C rate), good low-temperature
performance (86.6% capacity at -30$^\circ$C), and high safety, based on which,
a 56 V-11.5 Ah battery pack for E-bikes is successfully constructed, exhibiting
stable cycling (96.5% capacity at the 800th cycle) and a long driving distance
(36 km, tester weight 65 kg). This work offers a commercially feasible cathode
material for low-cost, high-voltage NIBs, paving the way for advanced NIBs in
power and stationary energy storage applications.
