## 1 Introduction  
Rechargeable lithium-ion batteries (LIBs) have been widely used in electronic devices due to the high power capacity, high energy density, long cycle life, etc., and they are now being used in electric vehicles (EVs). Safety and the driving range of EVs are possibly prime concerns for the regulatory authorities and consumers. Among the most important parts of batteries, the electrolyte is one of the main factors to affect the safety, cost, and capacity of LIBs. An electrolyte solution is composed of at least three essential constituents: lithium salt, additives, and a solvent or a mixture of solvents. These three parts interact with each other electrochemically and solid electrolyte interface (SEI) film formed during the initial charging and discharging cycles [1; 2; 3; 4]. The formation mechanism of the SEI layer is complicated and not well understood yet, but the formation of the SEI layer takes place on the anode surface. The stable interfacial chemistry between the electrode and electrolyte is generally believed to be tightly associated with the performance of the electrochemical devices. In order to discover the appropriate electrolyte composition of LIBs for EVs, understanding the development trends in the patent database is a useful method. Based on the information through exploring the development trends, companies/organizations can manage their R&D resources and strategies [5]. According to the report of the World Intellectual Property Organization (WIPO), 90-95% of the world's inventions can be found in patents. Furthermore, if patent documents are employed to assist research and development (R&D), the time for research can be shortened by up to 60%, and the cost can be lowered as much as 40% [6]. Patent analysis refers to the process of using patent search techniques to retrieve patent information relevant to the subject of study, so as to transform the huge and trivial patent data into systematic and valuable technological information [7; 8; 9]. Patent analysis is useful in predicting technology on the basis of information retrieved as it conceives patents as the original source of information and commercial value. Furthermore, the patent analysis produces numerical results to discover important technological fields. [PERSON] et al. conducted a patent analysis to give an overview of the patent landscape within the field of LIBs [10]. They used annual patent counts to analyze research trends of LIBs' component from the 1990-2010 years. [PERSON] et al. took the patent families counts as data to analyze the R&D activities of batteries from the 2000-2011 years [11]. [PERSON] et al. also used patent counts to analyze development status in the electric vehicles industry from the 1977-2000 years [12]. These authors use the change of annual patent counts to discover the development situation in a specific technology. The results can help people gain skill in the art of understanding the relationship between the period of time of concern and patent activities. However, it would be rather difficult to see the patent assignees' R&D strategies merely by a patent count analysis. Moreover, the interpretation of the development trends and the hidden intelligence behind the patent count analysis becomes still subjective. Using the patent indicator is another way for a patent analysis. [PERSON] used several patent indicators to analyze the patenting behavior of 50 business firms within the German mechanical engineering industry in 1995 [13]. These indicators included the counts of international patent applications, the rate of valid patents and patent citation counts. The result showed that a high indicator value is positively related to company profit performance. In 1998, [PERSON] further created new indicators to analyze the German, European and Japanese mechanical engineering companies to provide insights for strategic R&D planning [14]. These indicators include the relative patent position (RPP), technology attractiveness, etc.