1.9 Research Trends on Anodes for LIB 17
1.9.6 Metal Nitrides, Sulfides, Phosphides and Fluorides 41
Various limitations observed in the use of metal oxides as anodes for LIB, stimulated the need for investigations to explore other materials that could perform better. Thus, studies pertaining to binary/ternary metal-sulfides,-nitrides, -phosphides and -fluorides were undertaken [4,12]. The overall Li-uptake/removal for a binary metal compound MXm (X=F,O,S,N,P) is given by the following equation.
nLi + MXm ! M + nLiXm/n (1.12 ) where, m=n for X=F and m=n/2 for X=O,S
1.9.6.1 Metal fluorides
Maier and co-workers have examined the Li-recyclability of large number of metal fluorides (TiF3, VF3, MnF2, FeF2, CoF2, NiF2, CuF2) [165]. Initial discharge reaction involves LiF formation and structure destruction. Most of these compounds were found to have high decomposition voltage with respect to Li (> 2.0V).
1.9.6.2 Metal nitrides
Studies on the use of metal nitrides as anodes can be divided into two groups, viz., binary and ternary metal nitrides.
1.9.6.2.1 Binary nitrides
The nitrides studied for Li-cyclability include Sn3N4 [4], InN, Ge3N4 [4], Zn3N2 [4,166] and Cu3N [4] and Si/TiN composites [167]. The Sn3N4 and InN films are deposited by reactive sputtering of the respective metal in an atmosphere of Ar- N2 mixture. The reaction of the film with Li leads to the formation of LixSn/LixIn alloy embedded in a matrix of Li3N. Reversible capacity occurs due to the alloying- dealloying reaction of Sn/In while Li3N remains as passive matrix. As such these compounds exhibited capacity fading possibly due to inability of Li3N to serve as a good buffering medium.
The electrochemical reactions of Ge3N4, Zn3N2 and Cu3N are not as simple as those described for Sn- and In- nitrides. In the case of Ge3N4, only a part of the compound seems to undergo structure destruction and alloying while remaining portion remains unchanged. This gives rise to an ICL of 50 %. Subsequent cycles showed good reversibile capacity, ~ 450 mAh/g in the voltage range, 0.0- 0.7 V vs Li at a current of 23mA/g [4]. The reaction mechanisms in Zn3N2 and Cu3N are
complex: Zn3N2 undergoes complete structure destruction followed by alloy (LiZn) and Li3N formation during the first discharge; evidence for the LiZnN formation was also found. Zn3N2 also exhibited poor cyclability. In the case of Cu3N, reversible capacity arises due to the formation of Cu and Li3N up on deep discharge and reverse reaction occurring during the charge. Cu3N gave capacity values of ~ 300 mAh/g, stable up to 200 cycles between 0.0-3.0 V vs Li [4].
1.9.6.2.2 Ternary nitrides
Studies on ternary nitrides as anodes were carried out by Suzuki and Shodai [168] on Li7-xMnN4 and Nishijima et al. [169] on Li3FeN2. Li7MnN4 crystallizes in cubic antifluorite structure built of isolated MnN4 tetrahedra. The Mn and Li are located at tetrahedral sites and their orderly arrangement ensures 3-dimensional pathways for Li-diffusion. After initial delithiation cycle, the aforesaid compound cycles well at ~1.2V vs. Li with a reversible capacity ~300mAh/g accompanied with reversible structural transformations. In the case of Li3FeN2 with the orthorhombic structure, Fe4N tetrahedra share edges to form 1D- chains along the c direction.
Delithiation occurs at 1.2 V with the appearance of four phases, all having orthorhombic symmetry. Subsequent Li-insertion is a reversible process causing re- development of the phases [169].
The hexagonal Li2(Li1-xMx)N, M=Co, Cu, Ni, (Co,Cu) or (Co, Cu, Fe) being isostructural to Li3N were studied for their anodic response [4,170,171]. It was reported that Li2.6Co0.4N showed an initial capacity as high as 900 mAh/g in the voltage range, 0.1 to 1.5 V but it degraded on cycling. In the case of Li2.6(Co0.2Cu0.2)N, a reversible capacity of 600 mAh/g in the voltage range, 0-1.3 V remained stable up to 60 cycles. The phases with Ni and Fe partly replacing the Cu in the above compound were also found to exhibit satisfactory performance. The
investigations on ternary-metal-nitrides demonstrated their viability as LIB anodes.
However, they are highly moisture sensitive (hygroscopic). It appears that these compounds become amorphous upon first Li-extraction, making it difficult to understand the exact reaction mechanism.
1.9.6.3 Metal phosphides
Various metal phosphides have been investigated as prospective anodes for LIB: MnP4 [12], FeP2 [4], Li2CuP, Cu3P [172], CoP3 [4,173,174] and recently Sn4P3
[175]. The layered MnP4 compound comprises infinite sheets of MnP6 octahedra linked by P-P bonds both within the sheets and between the sheets. Li-insertion causes the formation of Li7MnP4, possessing the antifluorite structure [12]. This reaction is reversible and the original MnP4 phase can be restored upon Li- deintercalation. However, the reactions are accompanied by extensive unit cell volume variations leading to capacity fading. Observed initial reversible capacity was 700 mAh/g (theoretical ~ 1050 mAh/g).
The reaction mechanism for FeP2 [4] has been explained on the basis of XRD, XAS and magnetic measurements. First discharge cycle is associated with 6 moles of Li-insertion and a capacity of 1365 mAh/g leading to an amorphous phase with substantial Fe-P bonding. The XAS and EXAFS analyses showed that the end product is ‘Li-Fe-P’ and is devoid of a specific compound like Li3P. Thus, it was suggested that Li insertion causes the expansion of phosphorous network lattice and incorporation of additional Li leads to the formation of disordered, metastable
‘Li3Fe0.5P’ lattice. The following charge cycle causes de-insertion of 5.5 moles of Li and a capacity of 1250 mAh/g.
CoP3 forms nano-Co particles dispersed in Li3P matrix up on reaction with Li [4,173,174]. The charge reaction is associated with the decomposition of Li3P while
Co remains inactive. Thus, after first discharge cycle, the capacity arises by the reversible formation of Li3P from Li and P. However, another mechanism based on the transformation between Li3P and LiP has been proposed by another group of researchers [173]. In both the cases a capacity > 400 mAh/g has been reported which is well below the theoretical value.
Sn4P3 [175] has a layer structure with two non-equivalent phosphorous atoms located in the center of the tetragonal structure which are octahedrally surrounded by six tin atoms. Based on XRD, XAS and electrochemical data, a three-step reaction mechanism has been proposed for first discharge reaction of Sn4P3 with Li.
Step 1. Sn4P3 + 2Li+ + 2e- ( Li2Sn4P3 (1.13 a) Step 2. Li2Sn4P3 + Li+ + e- ( 4Sn + 3LiP (between 0.8-0.6 V) (1.13b) Step 3. Sn + 4.4Li+ + 4.4e- ! Li4.4Sn (below 0.6 V) (1.13c) LiP + 2Li+ + 2e- !Li3P
First charge cycle is the reverse of step 3, i.e., dealloying reaction of Li4.4Sn followed by conversion of Li3P to LiP. The compound showed good cyclability and retained 370 mAh/g up to 50 cycles in voltage range, 0.0-0.72 V.
1.9.6.4 Metal sulfides
Tin and iron sulfides were investigated as anodes for LIB [176,177]. The SnS2
powder made by sonochemistry and annealed at 400"C showed initial capacity values, 600 mAh/g in the voltage range, 0.0-2.0 V vs Li [177]. First discharge reaction is the structure destruction of SnS2 causing the formation of Sn-metal in a matrix of Li2S (Eqn. 1.14). Following charge-discharge cycling takes place by the Li-Sn alloy formation as per Eqn 1.7c.
SnS2 + 4.0 Li " Sn + 2Li2S (1.14)