  Electrodes in sodium-ion batteries endure mechanical stress during production
and application, which can damage these fragile coatings, causing performance
inefficiencies and early failure. Binder material provides elasticity in
electrode composites to resist fracture, but evaluating the effectiveness of
binder is complicated by substrate dependency of these films, while
conventional cell tests are beset by multiple electrochemical variables. This
work introduces a practical low-cost indentation test to determine the
elasticity of hard carbon electrodes containing standard carboxymethylcellulose
binder. Using the proposed method, relative elastic moduli of hard carbon
electrodes were found to be 0.079 GPa (1% binder), 0.088 GPa (2% binder), 0.105
GPa (3% binder) and 0.113 GPa (4% binder), which were validated using a
computational model of film deflection to predict mechanical deformation under
stress. Effects on the electrochemical performance of hard carbon anodes were
also demonstrated with impedance spectroscopy and galvanostatic cycling of
sodium half-cells, revealing 8-9% higher capacity retention of anodes with 4%
binder compared with those containing 1% binder. These findings suggest binder
content in hard carbon electrodes should be selected according to requirements
for both cycle life and film flexibility during cell manufacturing.
