batteries Review Reviews on the U.S. Patents Regarding Nickel/Metal Hydride Batteries Shiuan Chang 1, Kwo-hsiung Young 2,3,*, Jean Nei 3 and Cristian Fierro 3 1 Department of Mechanical Engineering, Wayne State University, Detroit, MI 48202, USA; [email protected] 2 Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202, USA 3 BASF/Battery Materials–Ovonic, 2983 Waterview Drive, Rochester Hills, MI 48309, USA; [email protected] (J.N.); cristian.fi[email protected] (C.F.) * Correspondence: [email protected]; Tel.: +1-248-293-7000 Academic Editor: Andreas Jossen Received: 21 February 2016; Accepted: 31 March 2016; Published: 12 April 2016 Abstract: U.S. patents filed on the topic of nickel/metal hydride (Ni/MH) batteries have been reviewed, starting from active materials, to electrode fabrication, cell assembly, multi-cell construction, system integration, application, and finally recovering and recycling. In each category, a general description about the principle and direction of development is given. Both the metal hydride (MH) alloy and nickel hydroxide as active materials in negative and positive electrodes, respectively, are reviewed extensively. Both thermal and battery management systems (BMSs) are also discussed. Keywords: metal hydride (MH); nickel/metal hydride (Ni/MH) battery; nickel hydride; electrode fabrication; U.S. patent 1. Introduction The nickel/metal hydride (Ni/MH) battery is an essential electrochemical device for consumer, propulsion, and stationary energy storages. Since its commercial debut in the late 1980s, many researchers have worked diligently in the Ni/MH battery field. Their contributions were publicized through two routes: academic publications and patent applications. While there are several key reviews available on the former [1–16], there is not one on the latter. Therefore, we have organized and summarized the patents related to Ni/MH battery technology in the current review and its companion—Reviews on the Japanese Patents Regarding Nickel/Metal Hydride Batteries [17]. 2. Results As shown in Figure1, the current review is divided into six categories in the chronological order of technology development, which are active materials Ñ electrode fabrication Ñ cell assembly Ñ multi-cell construction Ñ system integration Ñ application. Because of the extremely large number of U.S. patents on these subjects, we have selected and presented the most representative patents historically and technologically in the current review. More related U.S. patent documents can be found on each U.S. patent presented here. Batteries 2016, 2, 10; doi:10.3390/batteries2020010 www.mdpi.com/journal/batteries Batteries 2016, 2, 10 2 of 29 Batteries 2016, 2, 10 2 of 28 Figure 1.1. Content of this paper. MH:MH: metalmetal hydride.hydride. 2.1.2.1. Active Materials TheThe half-cellhalf-cell electrochemicalelectrochemical reactionreaction forfor thethe negativenegative electrodeelectrode is:is: − ⎯⎯→ − M + H2O + ´e ←⎯⎯ MH + OH´ (forward: charge, reverse: discharge) (1) M ` H2O ` e Ô MH ` OH pforward : charge, reverse : dischargeq (1) where M is a hydrogen storage alloy capable of storing hydrogen reversibly and MH is the wherecorresponding M is a hydrogenmetal hydride storage (MH). alloy During capable charge, of storingapplied hydrogenvoltage splits reversibly water into and protons MH is and the correspondinghydroxide ions. metal Driven hydride by the (MH).voltage During and diffusio charge,n caused applied by voltage the concentration splits water difference, into protons protons and hydroxideenter into ions.the bulk Driven of the by thealloy voltage and then and meet diffusion with caused electrons by thefrom concentration the current difference,collector, which protons is enterattached into to the a power bulk ofsupply. the alloy During and discharge, then meet protons with electrons at the alloy from surface the currentrecombine collector, with hydroxide which is attachedions in the to electrolyte. a power supply. Electrons During are injected discharge, into protons the circuit at the load alloy through surface the recombine current collector with hydroxide in order ionsto maintain in the electrolyte. charge neutrality Electrons in arethe injected negative into electrode. the circuit load through the current collector in order to maintainThe counter charge reaction neutrality for inthe the negative negative electrode electrode. is: The counter reaction for the negative electrode is: ⎯⎯→ Ni(OH)2 + OH− ←⎯⎯ NiOOH + H2O + e− (forward: charge, reverse: discharge) (2) ´ ´ NipOHq2 ` OH Ô NiOOH ` H2O ` e pforward : charge, reverse : dischargeq (2) where Ni(OH)2 is the active material going through +2/+3 oxidation state change during the electrochemical reactions. During charge, protons deprived of Ni(OH)2 move to the surface of the where Ni(OH)2 is the active material going through +2/+3 oxidation state change during the positive electrode because of the applied voltage and recombine with hydroxide ions in the electrochemical reactions. During charge, protons deprived of Ni(OH)2 move to the surface of electrolyte. In order to maintain charge neutrality, electrons are injected into the power supply the positive electrode because of the applied voltage and recombine with hydroxide ions in the through the current collector. During discharge, water at the surface of the positive electrode splits electrolyte. In order to maintain charge neutrality, electrons are injected into the power supply through into protons and hydroxide ions. Protons then enter into NiOOH and neutralize with electrons the current collector. During discharge, water at the surface of the positive electrode splits into through the load to complete the circuit with the negative electrode. Shown in the net reaction protons and hydroxide ions. Protons then enter into NiOOH and neutralize with electrons through Equation (3), charge/discharge reaction does not change the overall hydroxide concentration/pH the load to complete the circuit with the negative electrode. Shown in the net reaction Equation (3), value, which is different from the working principle of Ni/Cd battery Equation (4). charge/discharge reaction does not change the overall hydroxide concentration/pH value, which is ⎯⎯→ different from theM working+ Ni(OH) principle2 ←⎯⎯ ofMH Ni/Cd + NiOOH battery (forward: Equation charge, (4). reverse: discharge) (3) ⎯⎯→ Cd(OH)M 2` + Ni2Ni(OH)pOHq2 2 Ô←⎯MH⎯ `CdNiOOH + 2NiOOHpforward + 2H2 :O charge,(forward: reverse charge, : discharge reverse: qdischarge) (3)(4) Both the inventions and further developmental works of MH alloy and Ni(OH)2 are presented CdpOHq2 ` 2NipOHq2 Ô Cd ` 2NiOOH ` 2H2O pforward : charge, reverse : dischargeq (4) in the next two sections. Both the inventions and further developmental works of MH alloy and Ni(OH)2 are presented in the2.1.1. next Metal two Hydride sections. Alloys in the Negative Electrode 2.1.1.Klaus Metal Beccu Hydride contributed Alloys in theto the Negative earliest Electrode patent using MH alloy as an active material in the negative electrode to construct a Ni/MH battery in 1970 [18]. The alloy used had a composition of Klaus Beccu contributed to the earliest patent using MH alloy as an active material in the negative Ti85Ni10Cu3V2 (in wt%), which was later expanded to any hydride of the third, fourth, or fifth group electrode to construct a Ni/MH battery in 1970 [18]. The alloy used had a composition of Ti Ni Cu V of transition metal (for example, Ti) [19]. Two types of MH alloys were used 85in the10 first3 2 (in wt%), which was later expanded to any hydride of the third, fourth, or fifth group of transition commercialized Ni/MH batteries introduced in 1989. While Japanese companies such as Matsushita metal (for example, Ti) [19]. Two types of MH alloys were used in the first commercialized Ni/MH [20–22] (later changed its name to Panasonic), Toshiba [23], Sanyo [24,25], and Yuasa [26] used the batteries introduced in 1989. While Japanese companies such as Matsushita [20–22] (later changed its misch metal-based (mixtures of light rare earth elements such as La, Ce, Pr, and Nd) AB5 MH alloy, name to Panasonic), Toshiba [23], Sanyo [24,25], and Yuasa [26] used the misch metal-based (mixtures the Ovonic Battery Company (OBC, Troy, MI, USA) chose the transitional metal-based AB2 MH alloy of light rare earth elements such as La, Ce, Pr, and Nd) AB MH alloy, the Ovonic Battery Company as the active material in the negative electrode. General properties5 of these two alloy families are (OBC, Troy, MI, USA) chose the transitional metal-based AB2 MH alloy as the active material in the listed in Table 1. The AB5 MH alloy eventually dominated the market due to the substantially lower price of misch metal since 2000. Batteries 2016, 2, 10 3 of 29 negative electrode. General properties of these two alloy families are listed in Table1. The AB 5 MH alloy eventually dominated the market due to the substantially lower price of misch metal since 2000. Table 1. Property comparison between the AB2 and AB5 MH alloy families. FCC, 1C/1C, and DOD denote face-centered-cubic, 1C charge and discharge rates, and depth of discharge, respectively. Property AB2 MH alloy AB5 MH alloy Basic crystal structure Hexagonal C14 and FCC C15 Hexagonal CaCu5 Composition example Ti12Zr21Ni38V10Cr5Mn12Co1.5Al0.5 La10Ce5Pr0.5Nd1.5Ni60Co12Mn6Al5 Discharge capacity 340–440 mAh¨ g´1 300–330 mAh¨ g´1 High-rate dischargeability Acceptable Excellent Cycle life (1C/1C, 100% DOD) 800 1200 Self-discharge Acceptable (V-free alloy) Acceptable High-temperature storage Acceptable due to leach-out Bad due to surface passivation Activation Modest Easy Low temperature Good Modest Fabrication method Casting, hydriding, and grinding Casting and grinding Raw material cost V is expensive Volatile due to rare earth price fluctuation Low-cost version V and Co-free alloy Pr, Nd, and Co-free alloy Discovery of the hydrogen storage capability of the rare earth-based AB5 intermetallic alloy was accidental [27]. Guegen [28] filed the first patent using LaNi5 modified with Ti, Ca, Ba, Cr, and/or Cu with improved capacity as negative electrode material in 1978.