LiMn₂O₄ (LMO) is a promising cathode material for Li ion batteries due to its high voltage, environmental benignity, and natural abundance.^1,2 Currently, LMO suffers from capacity fade due to Mn ion dissolution and a Jahn-Teller distortion which reduces the stability of the cathode structure.^3,4 Mechanical thin film measurements can provide information on the mechanical deformations of LMO during cycling and how these deformations affect the stability and longevity of LMO.

The surface stress and bulk strain associated with the mechanical deformation of LiMn₂O₄ as a result of lithiation and delithiation is measured using an in situ bending cantilever technique and an in situ digital image correlation technique, respectively. The combination of these techniques allows for a comparison between bulk and surface changes in the cathode material in a slurry cast electrode. The composite electrodes consist of ball-milled LMO active material, carboxymethylcellulose sodium salt binder, and conductive Super P carbon black and were tested in 1 M LiPF₆ electrolyte in 1:1 ethylene carbonate/dimethyl carbonate.

Strain measurements show volume contraction upon delithiation and volume expansion upon lithiation of the LMO electrode. The derivative of strain with respect to potential exhibits four distinct features, two upon delithiation (see figure) and two upon lithiation. The main features are aligned with the peaks in the cyclic voltammogram (CV) and correlate to changes in the LMO spinel lattice parameters.⁵

Tensile stress (surface contraction) is observed upon LMO delithiation, and compressive stress (surface expansion) is observed upon lithiation. The derivative of stress with respect to potential also exhibits four distinct features. The stress derivative feature at lower potentials during delithiation precedes the current peak and therefore indicates that a significant surface contraction occurs before delithiation (see figure).

Stress and strain occur asynchronously during delithiation of LMO suggesting that different processes control bulk versus surface expansion and contraction. Strain is governed by changes in the bulk lattice parameters, and stress is controlled by numerous processes such as surface film growth and lattice distortions at the electrode/electrolyte interface.

 References

1. Thackeray, M. M.; Johnson, P. J.; Depicciotto, L. A.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1984, 19, 179.
2. Yi, T. F.; Zhu, Y. R.; Zhu, X. D.; Shu, J.; Yue, C. B.; Zhou, A. N. Ionics 2009, 15, 779.
3. Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461.
4. Amatucci, G. G.; Schmutz, C .N.; Blyr, A.; Sigala, C.; Gozdz, A. S.; Larcher, D.; Tarascon, J. M. J. Power Sources 1997, 69, 11.
5. Sun, X.; Yang, X. Q.; Balasubramanian, M.; McBreen, J.; Xia, Y.; Sakai, T. J. Electrochem. Soc. 2002, 149, A842.