Self-consistent proton-neutron quasiparticle random phase approximation based on the spherical nonlinear point-coupling relativistic Hartree-Bogoliubov theory is established and used to investigate the β + /electroncapture (EC)-decay half-lives of neutron-deficient Ar, Ca, Ti, Fe, Ni, Zn, Cd, and Sn isotopes. The isoscalar proton-neutron pairing is found to play an important role in reducing the decay half-lives, which is consistent with the same mechanism in the β decays of neutron-rich nuclei. The experimental β + /EC-decay half-lives can be well reproduced by a universal isoscalar proton-neutron pairing strength.Nuclear β decays play important roles in many subjects of nuclear physics. Specifically, the investigation of β decay provides information on the spin and isospin dependence of the effective nuclear interaction, as well as on nuclear properties such as masses [1], shapes [2], and energy levels [3]. Moreover, nuclear β decays are also important in nuclear astrophysics, because they set the time scale of the rapid neutron-capture process (r-process) [4][5][6][7][8], which is a major mechanism for producing the elements heavier than iron. In addition, nuclear β decays can provide tests for the electroweak standard model [9][10][11]. With the development of radioactive ion beam facilities, the measurement of nuclear β-decay half-lives has achieved great progress in recent years [12][13][14][15].On the theoretical side, apart from the macroscopic gross theory [16], two different microscopic approaches have been widely used to describe and predict the nuclear β-decay rates. They are the shell model [5] and the proton-neutron quasiparticle random phase approximation (QRPA) [17][18][19]. While the shell model takes into account the detailed structure of the β-strength function, the proton-neutron QRPA approach provides a systematic description of β-decay properties of arbitrarily heavy nuclei. In order to reliably predict properties of thousands of unknown nuclei relevant to the r-process, the self-consistent QRPA approach has become a current trend in nuclear structure study, including those based on the Skyrme-Hartree-Fock-Bogoliubov (SHFB) theory [20] and the covariant density functional theory (CDFT) [21][22][23].In the CDFT framework, the self-consistent proton-neutron RPA was first developed based on the meson-exchange relativistic Hartree (RH) approach [24]. To describe the spinisospin excitations in open shell nuclei, it has been extended to the QRPA based on the relativistic Hartree-Bogoliubov (RHB) approach [25] and employed to calculate the β-decay half-lives of neutron-rich nuclei in the N ≈ 50 and N ≈ 82 regions [21,22]. In addition, based on the meson-exchange * haozhao.liang@riken.jp relativistic Hartree-Fock (RHF) approach [26,27], the selfconsistent proton-neutron RPA has been formulated [28] and well reproduces the spin-isospin excitations in doubly magic nuclei, without any readjustment of the parameters of the covariant energy density functional [28,29]. Recently, the self-consistent QRPA b...