We report our electrochemical and in situ x-ray diffraction experiments on a variety of tin oxide based compounds; SnO, Sn02, Li2SnO3, and SnSiO3 glass, as cathodes opposite lithium metal in a rechargeable Li-ion coin cell. These materials demonstrate discharge capacities on the order of 1000 mAh/(g Sn), which is consistent with the alloying capacity limit of 4.4 Li atoms per Sn atom, or 991 mAh/(g Sn). These materials also demonstrate significant irreversible capacities ranging from 200 mAh/(g active) to 700 mAh/(g active). In situ x-ray diffraction experiments on these materials show that by introducing lithium, lithium oxide and tin form first, which is then followed by the formation of the various Li-Sn alloy phases. When lithium is removed the original material does not reform. The ending composition is metallic tin, presumably mixed with amorphous lithium oxide. The oxygen from the tin oxide in the starting material bonds irreversibly with lithium to form an amorphous Li20 matrix. The Li-Sn alloying process is quite reversible; perhaps due to the formation of this lithia "matrix" which helps to keep the electrode particles mechanically connected together.* Electrochemical Society Active Member.372 mAh/g X 2.2 g/ml (density of graphite) = 800 mAh/mL

Tin oxide composite glasses represent a new class of material for the anode of Li-ion cells. Using results of experiments on Li/Sn 2 BPO 6 and Li/SnO 2 cells, we identify those factors which are responsible for good charge-discharge capacity retention, First, the grains (those regions which diffract coherently) which make up the particles of the material should be as small as possible. Then, regions of tin which form are kept small and two-phase coexistence regions between bulk Li-Sn alloys of different composition do not occur. The Sn 2 BPO 6 glass represents the smallest grains possible. Second, the particles themselves should be small so that they can each be well contacted by carbon black during electrode manufacture. Third, the voltage range of cycling must be selected so that the tin atoms do not aggregate into large regions which grow in size. This aggregation is evidenced by the growth of peaks in the differential capacity vs. voltage as a function of cycle number. The peaks represent the coexistence between bulk Li-Sn alloy phases which 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 voltage profiles which do not change with cycle number (as indicated by a stable differential capacity) give the best cycling performance. The selection of the voltage limits for cycling strongly influences the stability of the voltage profile (as illustrated here), so this must be done with much care.

We show that the reaction mechanism in Li/[SnO:(B 2 O 3 ) x :(P 2 O 5 ) y glass (0.1 Յ x, y Յ 0.5)], Li/[SnO: B 2 O 3 ) 0.5 :(P 2 O 5 ) 0.5 :(K 2 CO 3 ) 0.04 glass] and Li/SnO cells is common. During the first discharge, the oxygen bonded to tin (as SnO) reacts with lithium to give metallic tin (which can be present as clusters of a few atoms) and lithia. The tin reacts with further lithium to the composition limit of Li 4.4 Sn. During charge the Li is removed from the lithium-tin alloy. The other components of the glass (e.g., B 2 O 3 , P 2 O 5 , Li 2 O, etc.) are inert with respect to lithium, and we call the atoms which make up these phases "spectator atoms." Using X-ray diffraction (XRD) and electrochemical methods, we show that size of the initial tin regions which form is inversely proportional to the spectator:Sn atom ratio. However, during cycling, all of these materials show the subsequent aggregation of the tin atoms into clusters which grow with cycle number until they reach a saturated size. The final cluster size is larger for materials with smaller X:Sn ratio. We propose a speculative model, which predicts the saturated Sn cluster size, as a function of the spectator:Sn ratio.

and Li͒ of the lithium-tin phase diagram. This information was used to determine a theoretical electrochemical voltage profile, which compares well with experiment for xϽ2.5 in Li x Sn. For xϾ2.5, it was found that the equilibrium structures predicted by the phase diagram were not formed in the room-temperature electrochemical cell, which explains why the calculated results are less good there. Calculations of this type are useful for materials research in lithium-ion batteries. ͓S0163-1829͑98͒03048-3͔

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