ADVANCES IN ELECTRODE MATERIALS RESEARCH

The technologies of primary (non-rechargeable) and secondary (rechargeable) lithium batteries have been developed in parallel to the developments in the microelectronics industry for about 20 years. The lithium batteries can be the lightest and most compact power sources when they are intended for portable equipment such as watches, calculators, cameras, cordless telephones, computers, and so on. During the past 10 years, the worldwide "green revolution" has requested a new, clear, and noiseless system of energy storage for replacing petrol. Rechargeable lithium batteries as high power and high energy density devices can also meet the requirements of clear and noiseless energy source, especially in the development of electric vehicles. Consequently, rechargeable lithium batteries with high power and long-life cycles have been very attractive to research groups all over the world. Thermodynamically, an electrochemical e.m.f, system is generated if an dectrolyte is sandwiched between two electrode materials with different chemical potentials. Electrodynamically, if a constant supply of ions can be maintained and transported through the electrolyte, this system will deliver current when connected across a load circuit. In the beginning of the research on rechargeable lithium batteries, a simple effect system has been studied. As a Li intercalation compound consists of a host (H) matrix into/from which x guest ions Li + may be inserted/extracted reversibly without a rearrangement of the host structure, a simple lithium battery may be represented as:


I. INTRODUCTION
The technologies of primary (non-rechargeable) and secondary (rechargeable) lithium batteries have been developed in parallel to the developments in the microelectronics industry for about 20 years.The lithium batteries can be the lightest and most compact power sources when they are intended for portable equipment such as watches, calculators, cameras, cordless telephones, computers, and so on.
During the past 10 years, the worldwide "green revolution" has requested a new, clear, and noiseless system of energy storage for replacing petrol.Recharge- able lithium batteries as high power and high energy density devices can also meet the requirements of clear and noiseless energy source, especially in the develop- ment of electric vehicles.Consequently, rechargeable lithium batteries with high power and long-life cycles have been very attractive to research groups all over the world.
Thermodynamically, an electrochemical e.m.f, system is generated if an dectrolyte is sandwiched between two electrode materials with different chemical potentials.Electrodynamically, if a constant supply of ions can be maintained and transported through the electrolyte, this system will deliver current when con- nected across a load circuit.In the beginning of the research on rechargeable lithium batteries, a simple effect system has been studied.As a Li intercalation compound consists of a host (H) matrix into/from which x guest ions Li + may be inserted/extracted reversibly without a rearrangement of the host structure, a simple lithium battery may be represented as: (1) If $A and kB represent the anode and cathode work functions respectively, the open-circuit voltage Voc can be expressed as: Voc (kA SB)/e0, *Present address: Institut de Chemic de la Matira Condens6e de Bordeaux, CNRS, Universit6 where e 0 designates the electronic charge.However, the use of lithium metal as the negative electrode in rechargeable cells introduces some problems.First, the dendritic deposition of lithium metal upon recharging causes internal short-circuits and occasionally may cause explosion of these cells.Second, the freshly deposited lithium dendrites react with the electrolyte and become isolated from the bulk lithium metal resulting in lower coulombic efficiencies of lithium electrodes and poor cycling life. 2 These problems can be circumvented if another lithium interca- lation compound (LiH) is used as the negative electrode instead of the limited rechargeability and the safety hazards of lithium metal.Therefore, a "double effect" system of so-called "rocking-chair" has been proposed since 1980; 3-8 it can be illustrated as: Anode o LixH/Li +/LiyH z Cathode. (3) The lithium intercalation potential of the anode material LixH1 is generally more negative than that of the cathode material LixH2.In fact, the potential Voc of the rocking-chair cells should be the difference between the chemical potential of lithium in the host structures (H 1) and (H2), respectively.Voc can also be estimated using equation (2); it is depicted in the schematic energy diagram shown in figure 1.For instance, Voc can be as high a 4 V for a "rocldng chair" battery using LixC as (H) and Lil_xCOO 2 as (H2). 9 Fig. 2 depicts some electrode potentials measured vs. a lithium electrode.
These performances obviously depend not only upon the structure and the texture of the electode materials, but also on the nature of the constitutive atoms.
In this paper, we firstly describe the structural and textural effects that influence the electrochemical properties of the electrode materials; then a correlation between the open-circuit voltage and the electronic energy configuration of the electrode materials is reported, accounting for their electrochemical behavior.* Compounds prepared and studied at the "Laboratoire de Chimie du SolMe-Universit de Bordeaux.

II.1. STRUCTURAL CONSIDERATIONS
The technology of lithium batteries has been developed on the basis of the lithium intercalating material research.In many cases, when the electrode is based on inorganic materials, its capacity, insertion rate of lithium, and reversibility are related to its crystal structure host.The classification of host structures is generally represented as: 9 layered hosts, ex" Lil_xMO2( M Co, Ni) and LixV205 ID runnels, ex" WO3-tetrag.and hexag.Host structures framework hosts 2D tunnels, ex" TiO2-anamse 3D tunnels, ex Li _xM204-spinels (M Ti, Mn) 1.a.Layered oxides: Lil_xMO 2 (M Co, Ni) and LiV205 are presented as illustrative examples.: In the layered oxides Lil_xMO 2 (M Co, Ni) (Fig. 3), the Li and M atoms occupy alternate (111) planes of the rocksalt structure.The O-M-O layers are bonded electrostatically via the interlayer Li + ions.The latter remain in octahedral coordination over the entire range of stability of the Li_MO2 system.Let us note that the research on LiCoO 2 is probably more advanced than for Lil_NiO 2 because of the simplicity of preparing the former material.However, the rechargeable capacity of LiCoO 2 is limited to less than 150 mAh g.llIn fact, the layered structure becomes more unstable as x -, 1 because the Van der Waals forces between the oxide-ion layers tend to be weaker than the electrostatic repulsive forces. 12Consequently, attempts are being done, at the present time, to stabilize the layered structure using appropriate dopings, such as polymer-based pillars between the layers.Another approach, so called "textural approach" presented in the next section, might also be advantageously used. 13'4-16 For instance, a 50% displacement of nickel ions at 3(a) sites and lithium ions at 3(b) sites gives a rock-salt type LiNiO 2, which is electrochemically inactive because of the immobile lithium ions at t.he octahedral sites in the matrix.On the other hand, a 25% regular displacement between 3(a) and 3(b) sites gives a spinel-framework structure in which lithium ions supposedly can move from one site to the other as in LiMn204 .7 T. Ohzuku et al succeeded in preparing electrochemically strongly active LiNiO 2 electrodes using proper processing methods.For instance, LiNiO 2 prepared from LiNO 3 and NiCO 3 or Ni(OH) 2, exhibited a rechargeable capacity as high as 200 mAh g-1 within 2.5 and 4.1 V vs. Li (corresponding to x 0.72 in Lil_xNiO2).
For instance, V603 can reversibly intercalate up to one Li per vanadium atom, and it is being developed widely in polymer prototype rechargeable Li batteries.As for the vanadium bronzes of general formula LixV205, their structure is strongly dependent upon both the synthesis conditions and the lithium content (x).The phase diagram of the Li-based vanadium oxide system, established by room temperature insertion of Li into the the pseudo-layered V205, consists of several phases denoted by a, e, and to-LixV_Os, 23'25 whose structures are closely related to that of V205. 26The to-LixV205 phase, which reversibly intercalates two lithium ions per V205 between 4 and 1.9V, has been proposed as a new cathode for rechargeable lithium batteries.27 The a phase (0 < x < 0.1 in LiV20 5) can also be stabilized from solid state-state reactions above 350C.Moreover, by increasing the amount of lithium above 0.1, one still gets, from solid state reactions, new phases such as the y-phase (fig.5).In fact, y-LiV20 5 exists within the range of composition 0.88 < x < 1; moreover, it has a structure 28 that allows for the electrochemical removal of lithium topotactically so that a new V205 phase was obtained and denoted as y'-LiV205.29This new phase was demonstrated to be a promising positive elec- trode material for rechargeable Li batteries (fig.6).x in LixV,_Os 2.0 0.0 0.5 1.0 1.5 2.0 Second and third discharge of a Li//-LixV205 cell between 3.8V and 2V at 0.25 1.b.Tunnel framework oxides: WO 3, TiO2, Lil_xM204 (M Ti, Mn) are presented as illustrative examples." The tetragonal and hexagonal tungsten-bronze structures contain isolated, parallel 1D tunnels into which the mobile ions are constrained.However, the 1D tunnels, as they occur in tetragonal and hexagonal WO a, are often blocked by stacking faults that hinder the ionic conductivity.Consequently, more attention has been given to ionic conductors having 2D and 3D tunnels.For instance, the lithium insertion rate, reversibly exchanged, can be as high as x 0.8 for TiO 2 anatase, which has a 2D tunnel structure. 13The cubic Lil_xM204 (M Ti, Mn) spinels offer interesting examples of a 3D tunnel structure: the M cations occupying half of the octahedral sites and the other empty octaheral sites, to which the Li + ions have access, form an interconnected 3D array of edge-shared sites. 12The dimen- sions of the spinel host are held rigid in 3D by strong M-O bonding, so it is possible to insert or extract Li + ions at room temperature to obtain Lil_xM204 (0 _< x < 1)without disrupting the [M2]O 4 array. 2'3 Let us note that the system Lil_xMn204 has also been investigated: a two-phase (cubic-tetragonal) region exists in the compositional range 0.1 _< x _< 0.8, so that the open-circuit voltage is 12 advantageously independant of x.
Finally, cathodes of composition Li+xMn204, which are synthesized using an original method (reduction of lithium-manganese-oxide and manganese-oxide pre- cursors with hydrogen at 300C to 350C, and with carbon at 600C) are even more tolerant to repeated lithium insertion and extraction. 31c.What would be the three most efficient cathode materials?: The above remarks show that the most promising cathode materials, for secondary lithium batteries of high specific energy, would be Lil_xMO2 (M Co or and Ni), Lil+xMn204, LixVEOs, LixV6Ol3 or their solid solution (i.e.: LilCo0.sNi0.502).

XTURAL CONSIDERATIONmNANOELECTROCHEMISTRY
The "NCIM (nano-crystallite insertion material) concept" has been recently ap- plied by some of us in the secondary lithium battery research and it may create a new research area so called "nanoelectrochemistry".
We have shown above that the electrochemical properties of the electrode materials, such as the rate of lithium reversible exchange, depend on their crystalline structure.This phenomenum can be called "structural effect".On the other hand, if the crystallites and particle sizes are decreased to nanoscale dimensions, an additional effect called "surface effect" intervenes.We are then dealing with the NCIM concept. 3e have reported elsewhere 3'32'33 that electrodes based on fine-grained transi- tion metal oxides (LixFe203, LiSnO2, Li2_xNiO2, etc.) exhibit efficient electro- chemical (de)insertion of Li + ions.The materials were symbolized as NCIMs.By minimizing the crystallite size, we favor, indeed, the formation of dangling and weak bonds at the surface and, thereby, the rate of lithium reversibly exchanged.

III. ELECTRONIC CONSIDERATION
During the electrochemical insertion, lithium ions and electrons are simultane- ously introduced into the electrode material.The corresponding inserted com- pound must be a good mixed ionic-electronic conductor.Good electronic conduc- tivity of the electrode is possible when the electronic charge transfers between two ions, M + and M+)+, which are on crystallographically, and thereby energeti- cally, equivalent sites.The carriers (electrons or holes) are either: (i) itinerant if a broad band is envolved in the conduction process or (ii) polaronic if their mobility carries an activation energy; however the activa- tion energy must be low enough in order to achieve the required high conductivity. 9,12e of these favorable situations is observed in the above quoted oxides Lil_xMO 2 (M Co, Ni) or Li_xMn204.'2 Therefore, when e g orbitals are involved in the conduction process, as it occurs for Lil_Mn204 or Li_NiO2, we are then dealing with a small polaron semiconductor (situation (ii)).A higher conductivity (than for Li_Mn20 or Li_NiO2) occurs for Li_CoO 2 and is related to a higher mobility; in Li_CoO 2 the hole carriers are delocalized in the broad w* band of C03//4/: r2s parentage.This band results from strong CO-Co interactions across shared octahedral site edges inducing a direct overlapping of the Co: t2s orbitals.0 C04+: 5 0 configurations place the Moreover, the (low spin) Co 3+" tses or tese Fermi energy E, which fixes the electrode potential at equilibrium, within the broad *(t2g) band located below the narrower r*(es) band.Consequently, the electrode potential is slightly higher for Li_CoO 2 than for Lil_xMll204 or Li_xNiO2.
Let us note that the top of the r* band lies well above the oxygen valence band of 2p parentage.Therefore, no mobile holes that would have caused the electrode decomposition are created in this band during the deintercalation process of Li + ions in Lil_xCOO 2. A fortiori, no mobile carriers occur in the oxygen valence band in the two other oxides Li_xMn20 4 and Li_xNiO 2 during the Li deintercalation process since the involved or*" e s band is located above the r* band and, therefore, above the 02: 2p 6 valence band.Such a favorable situation does not exist in the sulfides and selenides for which the top of the $2-: 3p 6 band lies at higher energy than the d orbitals of the M ions (M Mn, CO, Ni).Therefore, contrary to what is observed in the oxides, only the M 2+/3+ and not the M 4+ valence states are chemically accessible in the sulfides or selenides. 9'2 Consequently, the corresponding cathode materials will have a too-low potential vs Li and, therefore, have not drawn our attention in this paper.
Therefore, we have seen that there is a close relation between the electro- chemical and electronic properties of the electrode materials.Indeed, the rate of Li + ions that can be reversibly inserted into the electrode within given potential limits obviously depends upon the concentration and distribution of the energy states available for electrons. 34Related to that, the changes of cathode potential upon Li + insertion (such as those reported in fig. 4 for Lil_xCOO 2 and Lil_xNiO2) can correspond to Fermi level variations of the materials.Since the electronic structure of the electrode materials is connected with the overlap of the wave functions of electrons belonging to the transition metal atoms M, then the Fermi energy and therefore the electrode potential will depend on the M-M distance (R_).The latter generally increases with the increasing content of the lithium ions (thereby of the injected electrons).The increase in R_ makes theoverlap of the wave functions of 3d electrons diminish and at a critical M-M distance (Rc), it can bring about splitting of the effective energy band. 35Such an abrupt modification of the electronic structure might manifest itself in abrupt changes of the Fermi level and therefore of potential upon Li + insertion We have also seen that the discharge curve can present a steplike character when several successive phase transitions take place in the electrode material upon lithium insertion, as it occurs for Li +xMn204.However, the consideration of the discharge character depending on the electronic structure of the semicon- ducting electrode is still available because E changes during each phase transition.Fig. 7 illustrates schematically the expected character of the discharge curve in relation to the electronic structure of the cathode material for R_ < R and R_ R c.In the case of R c R_ O, high density of states near the Fermi level can lead to monotonous characters of the discharge curve (fig.7a).On the other hand, low density of states near E, as it occurs when (a) x in FIGURE 7 Expected character of discharge curve of cathode material for RM_ M < R (a) and RM_ M > R (b).TM R c -RM_ M < 0, can result in a strong compositional dependence of the elec- trode potential vs. x (fig.7b).

IV. CONCLUSION
It is preferable that the electrode materials have a host structure adapted to lithium intercalation, as it can occur for Li_xCoO2, Lil_xNiO2, Lil_Mn204, LiV205, or LiV603.However, ,when the crystallite size decreases to a nanomet- ric scale, the structural effect would be minimized in favor of the "textural effect," i.e., the electrochemical properties would then be influenced by the density of the surface states, the specific surface area, the grain boundary, etc.It has also been noted that potential changes observed during the electrochemical intercalation are, generally, those of the Fermi level of the cathode material. 8

FIGURE 1 FIGURE 2
FIGURE 1 Schematic energy diagram of a "rocking-chair" cell at open circuit.

FIGURE 3
FIGURE 3 Schematic representation of the Lil_xMO 2 layered structure. FIGURE4