KEY FACTORS CONTROLLING THE REVERSIBILITY OF THE REACTION OF LITHIUM WITH SNO2 AND SN2BPO6 GLASS

Tin oxide composite glasses represent a new class of material for the anode of Li-ion cells. Using results of experiments on Li/Sn2BPO6 and Li/SnO2 cells, we identify those factors which are responsible for goo d charge-discharge capacity retention. First, the grains (those region s which diffract coherently) which make up the particles of the materi al should be as small as possible. Then, regions of tin which form are kept small and two-phase coexistence regions between bulk Li-Sn alloy s of different composition do not occur. The Sn2BPO6 glass represents the smallest grains possible. Second, the particles themselves should be small so that they can each be well contacted by carbon black durin g electrode manufacture. Third, the voltage range of cycling must be s elected so that the tin atoms do not aggregate into large regions whic h grow in size. This aggregation is evidenced by the growth of peaks i n the differential capacity vs. voltage as a function of cycle number. The peaks represent-the coexistence between bulk Li-Sn alloy phases w hich have substantially different volumes. The coexistence is thought to cause fracturing and loss of contact between the grains. Therefore, materials with small particles, small grains, and smooth sloping volt age profiles which do not change with cycle number (as indicated by a stable differential capacity) give the best cycling performance. The s election of the voltage limits for cycling strongly influences the sta bility of the voltage profile (as illustrated here), so this must be d one with much care.