J.B. Bates

Thin-film lithium and lithium-ion batteries

Research over the last decade at Oak Ridge National Laboratory has led to the development of solid-state thin-film lithium and lithium-ion batteries. The batteries, which are less than 15 μm thick, have important applications in a variety of consumer and medical products, and they are useful research tools in characterizing the properties of lithium intercalation compounds in thin-film form. The batteries consist of cathodes that are crystalline or nanocrystalline oxide-based lithium intercalation compounds such as LiCoO2 and LiMn2O4, and anodes of lithium metal, inorganic compounds such as silicon–tin oxynitrides, Sn3N4 and Zn3N2, or metal films such as Cu in which the anode is formed by lithium plating on the initial charge. The electrolyte is a glassy lithium phosphorus oxynitride (‘Lipon’). Cells with crystalline LiCoO2 cathodes can deliver up to 30% of their maximum capacity between 4.2 and 3 V at discharge currents of 10 mA/cm2, and at more moderate discharge–charge rates, the capacity decreases by negligible amounts over thousands of cycles. Thin films of crystalline lithium manganese oxide with the general composition Li1+xMn2−yO4 exhibit on the initial charge significant capacity at 5 V and, depending on the deposition process, at 4.6 V as well, as a consequence of the manganese deficiency–lithium excess. The 5-V plateau is believed to be due to oxidation Mn of ions to valence states higher than +4 accompanied by a rearrangement of the lattice. The gap between the discharge–charge curves of cells with as-deposited nanocrystalline Li1+xMn2−yO4 cathodes is due to a true hysteresis as opposed to a kinetically hindered relaxation observed with the highly crystalline films. This behavior was confirmed by observing classic scanning curves on charge and discharge at intermediate stages of insertion and extraction of Li+ ions. Extended cycling of lithium cells with these cathodes at 25 and 100°C leads to grain growth and evolution of the charge–discharge profiles toward those characteristic of well crystallized films.

A Stable Thin‐Film Lithium Electrolyte: Lithium Phosphorus Oxynitride

The electrochemical and optical properties of lithium phosphorus oxynitride (Lipon) thin films have been studied with an emphasis on the stability window vs. lithium metal and the behavior of the Li/Lipon interface. Impedance measurements made between {minus}26 and 140 C show that Lipon exhibits a single, Li{sup +}-ion conducting phase with an average conductivity of 2.3 ({+-}0.7) {times} 10{sup {minus}6} S/cm at 25 C and an average activation energy of E{sub a} = 0.55 {+-} 0.02 eV. No detectable reaction or degradation was evident at the Li/Lipon interface, and linear sweep voltammetry measurements on three-electrode cells indicated that Lipon is stable from 0 to about 5.5 V with respect to a Li{sup +}/Li reference. The complex refractive index of Lipon was measured by spectroscopic ellipsometry. Optical bandgaps of 3.45 and 3.75 eV were obtained from the ellipsometry data and from optical absorption measurements, respectively.

Fabrication and characterization of amorphous lithium electrolyte thin films and rechargeable thin-film batteries

Amorphous oxide and oxynitride lithium electrolyte thin films were synthesized by r.f. magnetron sputtering of lithium silicates and lithium phosphates in Ar, Ar + O2, Ar + N2, or N2. The composition, structure, and electrical properties of the films were characterized using ion and electron beam, X-ray, optical, photoelectron, and a.c. impedance techniques. For the lithium phosphosilicate films, lithium ion conductivities as high as 1.4 × 10−6 S/cm at 25 °C were observed, but none of these films selected for extended testing were stable in contact with lithium. On the other hand, a new thin-film lithium phosphorus oxynitride electrolyte, synthesized by sputtering Li3PO4 in pure N2, was found to have a conductivity of 2 × 10-6 S/cm at 25 °C and excellent long-term stability in contact with lithium. Thin-films cells consisting of a 1 μm thick amorphous V2O5 cathode, a 1 μm thick oxynitride electrolyte film, and a 5 μm thick lithium anode were cycled between 3.7 and 1.5 V using discharge rates of up to 100 μA/cm2 and charge rates of up to 20 μA/cm2. The open-circuit voltage of 3.6 to 3.7 V of fully-charged cells remained virtually unchanged after months of storage.

Preferred Orientation of Polycrystalline LiCoO2 Films

Polycrystalline films of deposited by radio frequency magnetron sputtering exhibited a strong preferred orientation or texturing after annealing at 700°C. For films thicker than about 1 μm, more than 90% of the grains were oriented with their (101) and (104) planes parallel to the substrate and less than 10% with their (003) planes parallel to the substrate. As the film thickness decreased below 1 μm, the percentage of (003)‐oriented grains increased until at a thickness of about 0.05 μm, 100% of the grains were (003) oriented. These extremes in texturing were caused by the tendency to minimize volume strain energy for the thicker films or the surface energy for the very thin films. Films were deposited using different process gas mixtures and pressures, deposition rates, substrate temperatures, and substrate bias. Of these variables, only changes in substrate temperature could cause large changes in texturing of thick films from predominately (101)–(104) to (003). Although lithium ion diffusion should be much faster through cathodes with a high percentage of (101)‐ and (104)‐oriented grains than through cathodes with predominately (003)‐oriented grains, it was not possible to verify this expectation because the resistance of most cells was dominated by the electrolyte and electrolyte‐cathode interface. Nonetheless, cells with cathodes thicker than about 2 μm could deliver more than 50% of their maximum energies at discharge rates of or higher.

Electrical properties of amorphous lithium electrolyte thin films

The impedance of xLi2O·ySiO2·zP2O5 thin films deposited by RF-magnetron sputtering was analyzed using two models in which the frequency dependence of the bulk response was represented by: (1) a Cole-Cole dielectric function and (2) a constant phase angle element. Increases in the conductivity with Li2O concentration and with addition of SiO2 to Li2O-P2O5 compositions are attributed to an increase in Li+ mobility caused by changes in the film structure. A new amorphous oxynitride electrolyte, Li3.3PO3.9N0.17, prepared by sputtering Li3PO4 in N2, has a conductivity at 25°C of 2×10−6S/cm and is stable in conta with lithium.

Neutron irradiation effects and structure of noncrystalline SiO2

The nature of the defects produced in noncrystalline SiO2 and α‐quartz single crystals by fast neutron irradiation (E > 0.18 MeV) has been investigated by Raman and infrared spectroscopy, small‐angle x‐ray scattering, and electron microscopy. Measurements were made at 25°C on samples irradiated to integrated fluxes (exposures) from ∼1×1018 to 2×1020 neutrons/cm2 (n/cm2). It was observed that at 2×1020 n/cm2, the α‐quartz sample had been transformed to an essentially noncrystalline material. However, at 9×1019 n/cm2, remnant crystallites of the original α‐quartz having a radius of gyration of 47 A were found. The spectra of the noncrystalline phase of these materials are interpreted in terms of local modes at defect centers. Simple models are considered which account for the vibrational spectra and extrapolated x‐ray scattering in the forward direction. The post‐irradiation annealing behavior of the α‐quartz samples has been followed to 1000°C. It was found that the structure of the noncrystalline phase gr...

Rechargeable thin-film lithium batteries

Rechargeable thin-film batteries consisting of lithium metal anodes, an amorphous inorganic electrolyte, and cathodes of lithium intercalation compounds have been fabricated and characterized. These include LiTiS2, LiV2O5, and LiLixMn 2O4 cells with open circuit voltages at full charge of about 2.5 V, 3.7 V, and 4.2 V, respectively. The realization of these robust cells, which can be cycled thousands of times, was possible because of the stability of the amorphous lithium electrolyte, lithium phosphorus oxynitride. This material has a typical composition of Li3.3PO3.8N0.22 and a conductivity at 25°C of 2 microS/cm. The thin-film cells have been cycled at 100% depth of discharge using current densities of 5 to 100 microA/cm2. Over most of the charge-discharge range, the internal resistance appears to be dominated by the cathode, and the major source of the resistance is the diffusion of Li+ ions from the electrolyte into the cathode. Chemical diffusion coefficients were determined from ac impedance measurements.

Nanocrystalline Li{sub x}Mn{sub 2{minus}y}O{sub 4} cathodes for solid-state thin-film rechargeable lithium batteries

Thin-film cathodes of lithium manganese oxide, 0.3-3 μm thick, were deposited by rf magnetron sputtering of a LiMn 2 O 4 ceramic target onto unheated substrates. The resulting films were dense, ∼4.2 g/cm 3 , with a ∼50 A nanocrystalline spinel structure. The film composition was typically Li x Mn 2-y O 4 with y ∼ 0.3 and 1.2 < x < 2.2. When cycled in a thin-film rechargeable lithium battery, specific cathode capacities of 145 ± 23 and ∼270 mAh/g were realized for discharge from 4.5 V to either 2.5 or 1.5 V, respectively. The discharge and charge current densities were limited by the resistivity of lithium transport into and through the cathode. After thousands of cycles at 25°C, there was a small increase in cell resistance. After several hundred cycles at 100°C, the discharge curves developed a stable knee at ∼4 V characteristic of crystalline LiMn 2 O 4 cathodes. The polarization of the discharge/charge cycles were interpreted in terms of free energy of mixing curves.

Raman and Infrared Spectral Studies of Anhydrous Li2CO3 and Na2CO3

The infrared and Raman spectra of anhydrous crystalline Li2CO3 and Na2CO3 were measured at 300 and at 80°K. Bands observed in the vibrational spectra of Li2CO3 were assigned according to the C2h factor group symmetry. The doublet structure observed for each of the internal modes in the spectra of Na2CO3 was interpreted in terms of an ordered arrangement of CO3= ions over two nonequivalent orientations within the primitive cell. Multiple internal reflection and polarized specular reflectance techniques were used to determine transverse optical (TO) and longitudinal optical (LO) mode frequencies in Li2CO3 and Na2CO3. It was shown from these measurements that the infrared transmission spectra of these compounds exhibit band maxima which are admixtures of LO and TO modes.

Ionic conductivities and structure of lithium phosphorus oxynitride glasses

Abstract Lithium phosphorus oxynitride glasses with different lithium contents have been prepared by melting base glasses at high temperature in a flowing ammonia atmosphere for 16–72 h. The melt was then furnace-cooled to room temperature to avoid bubble formation in the sample. The structure of the lithium phosphorus oxynitride glasses was probed by X-ray photoelectron spectroscopy and high performance liquid chromatography. The results of ac impedance measurements show that nitrogen incorporation into the glass structure increases the ionic conductivity. The highest conductivity was found in Li0.99PO2.55N0.30 glass with σ ∼ 3.0 × 10−7 S cm−1 at 25°C and Ea = 0.60 eV.