*Review* **Metal Fluorides as Lithium-Ion Battery Materials: An Atomic Layer Deposition Perspective**

**Miia Mäntymäki 1,2,\*, Mikko Ritala <sup>2</sup> and Markku Leskelä <sup>2</sup>**


**\*** Correspondence: miia.mantymaki@picosun.com; Tel.: +358-50-320-6435

Received: 9 June 2018; Accepted: 6 August 2018; Published: 8 August 2018

**Abstract:** Lithium-ion batteries are the enabling technology for a variety of modern day devices, including cell phones, laptops and electric vehicles. To answer the energy and voltage demands of future applications, further materials engineering of the battery components is necessary. To that end, metal fluorides could provide interesting new conversion cathode and solid electrolyte materials for future batteries. To be applicable in thin film batteries, metal fluorides should be deposited with a method providing a high level of control over uniformity and conformality on various substrate materials and geometries. Atomic layer deposition (ALD), a method widely used in microelectronics, offers unrivalled film uniformity and conformality, in conjunction with strict control of film composition. In this review, the basics of lithium-ion batteries are shortly introduced, followed by a discussion of metal fluorides as potential lithium-ion battery materials. The basics of ALD are then covered, followed by a review of some conventional lithium-ion battery materials that have been deposited by ALD. Finally, metal fluoride ALD processes reported in the literature are comprehensively reviewed. It is clear that more research on the ALD of fluorides is needed, especially transition metal fluorides, to expand the number of potential battery materials available.

**Keywords:** atomic layer deposition (ALD); lithium-ion batteries; fluoride; thin films
