Development of sputtered nitrogen-doped Li_1+xAl_xGe_2-x(PO₄)₃ thin films for solid state batteries

Highlights

• N-doped Li_1+xAl_xGe_2-x(PO₄)₃ thin films were prepared by sputtering in a mixture of Ar + N₂ using an LAGP-powder target.

• The microstructure and properties of the films were characterized to understand how nitrogen is incorporated to the films.

• The entire process was optimised especially in terms of nitrogen content for high ionic conductivities in the films.

• A conductivity value of 2.3 × 10⁻⁴ S cm⁻¹ at 20 °C (2.7 times higher than undoped films) was achieved for N-doped LAGP films.

Abstract

Nitrogen-doped Li_1+xAl_xGe_2-x(PO₄)₃ (LAGP) thin films were prepared by magnetron sputtering in a mixture of Ar + N₂ using an LAGP powder target. The as-deposited films were amorphous, but could be crystallised into the NASICON LAGP phase after annealing at temperatures above 550 °C. The introduction of nitrogen to the sputtering gas has two effects on the deposited films; incorporation of a low concentration of nitrogen into the LAGP phase but also reduction of the rate of deposition. The former leads to improvements in Li ion conductivity whereas the latter can cause porosity and discontinuities in the films and limits their application in solid state devices. Up to 23% nitrogen in the sputtering gas the first effect is dominant and the total ionic conductivity improves without introducing morphological defects. However if the nitrogen content is increased further, the porosity decreases the measured conductivity. Optimised nitrogen doping in the sputtered LAGP films results in ionic conductivities as high as 2.3 × 10⁻⁴ S cm⁻¹ in films only 1 μm thick (2.7 times higher than undoped LAGP films) and activation energies below 0.38 eV.

Introduction

Solid-state lithium batteries may offer a solution to the safety issues being experienced in current lithium ion batteries containing flammable organic electrolytes [1,2]. The properties of the solid electrolyte is the key to the overall performance of these batteries, and the NASICON-type glass-ceramic Li_1+xAl_xGe_2-x(PO₄)₃ (LAGP) is a promising solid electrolyte being considered for this role because of its relatively high ionic conductivity, reasonably wide potential window, chemical stability and adequate mechanical properties [3,4]. Bulk LAGP electrolyte ceramics are normally fabricated by solid-state processing strategies resulting in room temperature ionic conductivity values in the range 3.3–6.7 × 10⁻⁴ S cm⁻¹ [3]–[6]. However it is difficult using solid-state processing to prepare the large area electrolyte layers thinner than 10 μm required to achieve high overall energy densities, and so thin film deposition methods have recently been explored to manufacture the thinner layers of solid electrolytes needed for optimising battery performance [7]. Sputtering has been used to fabricate thin films of several solid electrolyte materials, in particular LiPON [8], LLZO [9], and LAGP [10,11]. Recently we optimised the sputtering and post annealing parameters to fabricate 1 μm LAGP films with ionic conductivities as high as 10⁻⁴ S cm⁻¹ [12], comparable to those reported for bulk ceramic samples fabricated by solid-state processing at high temperatures [3,4].

It is well known that the addition of nitrogen to solid electrolyte compounds can decrease the activation energy for ionic diffusion [13,14], and this approach was successfully used to incorporate N into thin-film LiPON electrolytes by reactive RF magnetron sputtering from lithium orthophosphate (Li₃PO₄) targets [15]. The ionic conductivity of these films significantly increases with the atomic percentage (at.%) of N incorporated in the material, while the activation energy for Li diffusion correspondingly decreases. Bates et al. showed a substantial increase in ionic conductivity at 25 °C when the N content increases from 0 at. % (σ = 7 × 10⁻⁸ S cm⁻¹) to the highest concentration achieved of 6 at. % (σ = 3.3 × 10⁻⁶ S cm⁻¹) [15]. Based on the interpretation of XPS results from nitrogen-containing bulk sodium metaphosphate glasses, similar analysis of these LiPON films indicated the presence of two nitrogen environments that were assigned to nitrogen bound to either two or three phosphate tetrahedra [15,16]. This assignment has been widely accepted, and it has been proposed that the conductivity enhancement by nitrogen doping is due to the bridging of phosphate tetrahedra into a more strongly cross-linked glassy structure [17].

However, Wang et al. produced crystalline LiPON with the γ-Li₃PO₄ structure and interpreted their XPS data as showing that the majority of the N was bound to only one phosphate tetrahedron, with a minor contribution from Li – P disorder resulting in N bound to two tetrahedra. The N_1s XPS spectrum from this material appears similar to that reported by Bates et al., which raises questions about the validity of the original peak assignment [15,18]. Additionally, sputtered LiPON films have very different starting materials and processing conditions to bulk glasses, and a direct comparison between XPS spectra from the two may not be appropriate [19]. Lacivita et al. investigated nitrogen-incorporation in LiPON through combined experimental and computational methods and also determined that the nitrogen is either apical or bridging, and in these locations being covalently bonded to one or two phosphate groups respectively [20]. They suggested that these bridging nitrogen atoms are covalently bonded to two phosphorus atoms and interact less strongly with the mobile Li⁺ ions than apical nitrogen or oxygen atoms.

Since the development of LiPON, reactive nitrogen sputtering of electrolytes has been explored for other systems, including LiBON [21], LiSON [22], LiSPON [23], LiBPON [17,24], LiSiPON [25,26] and LiAlTiPON [27]. LATP thin films have been fabricated by magnetron sputtering in nitrogen, giving ionic conductivities as high as 1.22 × 10⁻⁵ S cm⁻¹ at room temperature [27]. A similar XPS analysis to that described above was used to suggest that the high ionic conductivity of these films is due to the formation of double and triply coordinated nitrogen in the phosphate network and the mixed anion effect which reduces the electrostatic energy [27]. LiSiPON thin films display an even higher ionic conductivity of 2 × 10⁻⁵ S cm⁻¹ (E_a = 0.45 eV) at room temperature, which is attributed to a combination of mixed-former, amorphization and nitridation effects [26].

Here we focus on the fabrication of polycrystalline N-doped LAGP thin films. The microstructure and ionic conductivity of these films have been characterized to provide an understanding of the effects of nitrogen doping on the microstructure and ionic conductivity in LAGP(N) films.